Syrian President Bashar Assad has vowed to continue a bloody crackdown on opposition groups within the country, saying they are killing civilians on a daily basis.

"The role of the government is to fight those militants in order to restore stability and to protect civilians," he said in an interview with Britain's Sunday Times.

"We have to prevent militants from doing what they are doing now, killing civilians, doing massacres, in different places in Syria," he added.

The UN says more than 3,500 people have been killed since the start of the uprising in mid-March. Assad on Sunday said over 800 members of Syria's security forces have also been killed.

Activist groups said Sunday at least three people were killed in continuing operations by security forces, including two in the flashpoint central city of Homs and one in northern Syria.

Meanwhile, there were unconfirmed reports of an attack on a building belonging to the ruling Baath party. Activists said the structure was hit by several rocket-propelled grenades.

The Free Syrian Army, a group of military defectors, claimed responsibility for the attack, saying it had inflicted damage and casualties.

However, witnesses reported the building appeared to be intact and there was no increase in the security presence around it.

Damascus-based journalist Thabet Salem, who lives about a kilometre away from the Baath party building and heard the explosions, said if the reports are confirmed, it would signal a new phase in the Syrian uprising.

"It would be an escalation that gives a new dimension to the whole situation," he said. International pressure on the Assad regime to put an end to the violence continues to grow.

Minister attacks Arab League

The Arab League, which suspended Syria from the organization last week, rejected proposed amendments to its peace plan by the embattled country, which would see a 500-member observer mission sent to the country.

Assad had agreed to the proposal in principle on Friday.

The league's plan also provides for the withdrawal of the government's tanks from the streets, the release of political prisoners and a halt to attacks on civilians.

Syria's foreign minister, Walid al-Moallem, attacked the Arab League on Sunday, saying it was being used as a "tool" to take the crisis to the UN Security Council.

He also said its peace plan gave excessive authority to observers and violated the country's sovereignty.

He said Syria would send a reply to the league, seeking clarification on why the amendments were rejected.

Peace deadline expires

Meanwhile, a deadline to end the violence, backed by threats of economic sanctions from the Arab League, passed overnight.

Turkey, a long-time ally of Syria, has recently become one of its most vocal critics.

On Saturday, Turkish Prime Minister Recep Tayyip Erdogan accused Syria of not fulfilling promises for reform or to stop the bloodshed.

Syria's uprising against Assad has grown more violent and militarized in recent weeks, as frustrated protesters see the limits of peaceful action.

Army dissidents who sided with the protests have also grown more bold, fighting back against regime forces and even assaulting military bases.

The increase in violence has many concerned the country is descending into civil war.

Syrian troops stormed a central town and a northwestern region in search of regime opponents on Saturday killing at least 15 people, activists said.