BEIRUT - A U.N. report on Syria said Tuesday there are "reasonable grounds" to believe that limited quantities of toxic chemicals have been used as weapons in at least four attacks in Syria's civil war, but said more evidence is needed to determine the precise chemical agents used or who used them.

The U.N. Commission of Inquiry said conclusive findings can be reached only after testing samples taken directly from victims or the site of the alleged attacks. It called on Damascus to allow a team of experts into the country.

The commission's report to the Human Rights Council on violations in Syria's conflict accused both sides of committing war crimes. In an apparent message to European countries considering arming Syrian rebels, the report warned that the transfer of arms would heighten the risk of violations, leading to more civilian deaths and injuries.

"War crimes and crimes against humanity have become a daily reality in Syria where the harrowing accounts of victims have seared themselves on our conscience," the report said. "There is a human cost to the increased availability of weapons," it added.

U.N.-Secretary General Ban Ki-moon has appointed a U.N. team to investigate alleged chemical weapons attacks in Syria after the Syrian government asked him to investigate a purported attack by rebels on March 19 on Khan al-Assal village in the northern city of Aleppo. But the Syrian government insists that a probe be limited to that incident.

Syrian soldiers were reportedly killed and injured in the incident, which the rebels blame on Syrian forces. Opposition activists have claimed more than six instances when regime forces used chemical weapons.

Ban is insisting on a broader investigation, including a December incident in Homs. He appointed Swedish chemical weapons expert Ake Sellstrom to lead a U.N. investigation. Syria has refused to allow his team into the country.

The confirmed use of chemical weapons could escalate the international response to the more than two-year-old conflict, which has killed more than 70,000 people, according to the United Nations. President Barack Obama has said their use would be a "red line," but the administration says it still looking for solid evidence.

"There are reasonable grounds to believe that chemical agents have been used as weapons," the report said. "It has not been possible, on the evidence available, to determine the precise chemical agents used, their delivery systems or the perpetrator."

The report said there are allegations of government forces using chemical weapons in four instances, but also did not rule out rebels using them.

"It is possible that anti-government armed groups may access and use chemical weapons.... though there is no compelling evidence that these groups possess such weapons or their requisite delivery systems," the report said.

"Conclusive findings - particularly in the absence of a large-scale attack - may be reached only after testing samples taken directly from victims or the site of the alleged attack," it said.

The report, covering the period from mid-January to mid-May, accused both sides of committing war crimes. On the government side, the report accused government forces and affiliated militia of committing torture, rape, forcible displacement and enforced disappearance. On the rebel side, the report accused armed groups of carrying out sentencing and execution without due process, as well as committing torture, taking hostages and pillaging.

But it said violations and abuses by the rebels "did not, however, reach the intensity and scale of those committed by government forces and affiliated militia."

"A dangerous state of fragmentation and disintegration of authority prevails in areas under anti-government armed groups control, despite attempts to fill the vacuum left by the withdrawal of the state through creating local councils," it said.

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  • March 2011: 120

    Thousands of Syrians rally to show their support for President Bashar al-Assad, who is facing unprecedented domestic pressure amid a wave of dissent, in Damascus on March 29 2011. (ANWAR AMRO/AFP/Getty Images)

  • April 2011: 820

    A protestor burns a portrait of Syrian President Bashar al-Assad during a demonstration after Friday prayers on April 29 2011 in Istanbul against the regime of al-Assad and the deadly crackdown on opposition protests. (BULENT KILIC/AFP/Getty Images)

  • May 2011: 850

    A veiled woman takes part in a protest calling on Syria's President Bashar Assad to step down, in front of the United Nations headquarters in Amman, on May 21 2011. (KHALIL MAZRAAWI/AFP/Getty Images)

  • June 2011: 1,000

    Syrian refugees arrive to a makeshift camp in the northern city of Idlib, in Syria, on June 13 2011. (MUSTAFA OZER/AFP/Getty Images) <em><strong>CORRECTION:</strong> An earlier version of this caption placed the city of Idlib in Turkey. Idlib is in Syria. </em>

  • July 2011: 1,600

    Thousands of pro-regime Syrians wave their national flag and portraits of President Bashar al-Assad during a rally in Damascus on July 17 2011. (LOUAI BESHARA/AFP/Getty Images)

  • August 2011: More than 2,000

    People hold pictures of Syrian president Bashar al-Assad and fallen Libyan strongman Muammar Gaddafi as they take part in a demonstration gathering activists opposed to Syria's regime of President Bashar al-Assad on August 28 2011 at Taksim Square in Istanbul. (BULENT KILIC/AFP/Getty Images)

  • September 2011: 2,700

    A national flag hangs on a statue of Syria's late president Hafez al-Assad at the entrance of the flashpoint city of Homs on August 30 2011, as rights activists reported widespread anti-regime protests across Syria on the first day of the feast marking the end of Ramadan. (JOSEPH EID/AFP/Getty Images)

  • October 2011: 3,000

    Supporters of Syrian President Bashar al-Assad wave Syrian flags during a pro-regime rally in Damascus on October 12, 2011. Assad's regime is facing international pressure amid a violent crackdown on anti-government protests that broke out in March across Syria. (LOUAI BESHARA/AFP/Getty Images)

  • November 2011: More than 4,000

    Syrian soldiers carry on November 26, 2011 the coffin of a comrade reportedly killed in an ambush by an armed group in the flashpoint Syrian city of Homs. (AFP/Getty Images)

  • December 2011: More than 5,000

    Free Syrian Army captain identified as Ahmed al-Arabi sits in a safe house near Wadi Khaled on the Lebanese-Syrian border on December 30 2011. (Si Mitchell/AFP/Getty Images)

  • January 2012: 7,100

    Free Syrian Army fighters take position in a house on the Lebanese-Syrian border prior to a nighttime operation on January 2 2012. (Si Mitchell/AFP/Getty Images)

  • February 2012: 7,500

    A Turkish journalist in Ankara, holds pictures of two journalists, French photojournalist Remi Ochlik and Sunday Times correspondent Marie Colvin, killed in an alleged rocket attack by Syrian regime forces against a makeshift opposition media center in the besieged city of Homs in Syria on February 22 2012. (ADEM ALTAN/AFP/Getty Images)

  • March 2012: More than 8,000

    A defected Syrian soldier, now a member of the Free Syrian Army, stands outside a mountain outpost near the village of Janudieh in the northern province of Idlib on March 20 2012. (STR/AFP/Getty Images)

  • April 2012: Close to 9,000

    Syrians pray over the bodies of Syrian violence victims at a funeral in the northwestern town of Kafr Zeta on April 10, 2012. (AFP/Getty Images)

  • May 2012: More than 9,000

    A general view shows the Syrian flag flying next to destruction in the Bab Amro neighbourhood of Homs on May 2 2012. (JOSEPH EID/AFP/GettyImages)

  • June 2012: 14,000

    A Syrian man carries a wounded girl next to Red Crescent ambulances following an explosion that targeted a military bus near Qudssaya, a neighbourhood of the Syrian capital, on June 8, 2012. (AFP/GettyImages)

  • July 2012: 19,000

    Members of Jihadist group Hamza Abdualmuttalib train near Aleppo on July 19, 2012. Rebels seized control of all of Syria's border crossings with Iraq on July 19. (BULENT KILIC/AFP/GettyImages)

  • August 2012: 23,000

    A Syrian boy whose family has been displaced due to fighting between rebel fighterws and Syrian government forces is seen near the Syrian border with Turkey on August 25, 2012. (ARIS MESSINIS/AFP/GettyImages)

  • September 2012: 30,000

    A Syrian man carrying grocery bags tries to dodge sniper fire as he runs through an alley near a checkpoint manned by the Free Syria Army in the northern city of Aleppo on September 14, 2012. (MARCO LONGARI/AFP/Getty Images)

  • October 2012: 36,000

    A Syrian boy plays on a destroyed tank near the rubble of a mosque that was destroyed during fighting between Syrian rebels and regime forces in the northern city of Azaz on September 23, 2012. (MIGUEL MEDINA/AFP/GettyImages)

  • November 2012: 40,000

    A walnut tree stripped of its branches stands in the rubble of the Kalat al-Numan citadel, originally built during the Roman era some 2000-years-ago, after allegedly being bombed several times by the Syrian air force on November 18, 2012, in Maaret Al-Numan in southern Idlib province. (John Cantlie/AFP/Getty Images)

  • December 2012: 60,000

    Smoke rises in the Hanano and Bustan al-Basha districts in the northern city of Aleppo on December 1, 2012 as fighting continues through the night. (Javier Manzano/AFP/Getty Images)

  • January 2013: 65,000

    A boy plays with a balloon in a Syrian refugees camp in Azaz, near the Turkish border, on January 10, 2013 after snow falls. (EDOUARD ELIAS/AFP/Getty Images)

  • February 2013: More than 70,000

    Syrian Zakia Abdullah sits on the rubble of her house in the Tariq al-Bab district of the northern city of Aleppo on February 23, 2013. (Pablo Tosco/AFP/Getty Images)