In an impromptu press conference in Geneva, Annan told reporters Thursday he was dismayed by the lack of progress in enforcing a Syria ceasefire, but had accepted "the daunting challenge" of trying to negotiate peace because of the mounting humanitarian cost.
"I accepted this task, which some called 'mission impossible,' because I believed it was a sacred duty to do whatever was in my power to help the Syrian people find a peaceful solution to this bloody conflict," he said. His resignation comes amid escalating violence in Syria.
"The increasing militarization on the ground and the clear lack of unity in the Security Council have fundamentally changed the circumstances for the effective exercise of my role."
Annan said most of the blame falls on the Syrian government for its continued refusal to implement his six-point plan for peace, which included calls for Syria to commit to a ceasefire, an immediate stop to the use of heavy weapons in populated areas, a daily two-hour halt to fighting to allow for the evacuation of injured and the delivery of aid, and Syrian-led talks aimed at finding a political solution.
He also said bickering between the five veto-wielding nations in the 15-member Security Council over the stalemate made succeeding in his task "impossible."
"At a time when the Syrian people desperately need action, there continues to be finger-pointing and name-calling in the Security Council," he said.
"It is impossible for me or anyone to compel the Syrian government and also the opposition to take the steps to bring about the political process."
Annan, who was served as the special envoy since last February, told reporters at a press conference in Geneva on Thursday he shared in the international community's frustrations with the lack of progress in enforcing a Syria ceasefire.
UN Secretary General Ban Ki-moon announced Annan's resignation Thursday "with deep regret," saying in a statement that Annan has no intention to renew his mandate when it expires on Aug. 31.
"I wish to express my deepest gratitude to Mr. Annan for the determined and courageous efforts he has made as the joint special envoy for Syria," Ban said, adding that consultation is underway with the League of Arab States for "the prompt appointment of a successor."
Asked about the succession plans, Annan said, "The world is full of crazy people like me, so don't be surprised if someone else decides to take it on."
Canada's Foreign Affairs Minister John Baird also lauded Annan's "noble and tireless" efforts to make peace in Syria in the face of the violent Assad regime.
"Canada is not overly surprised to learn of this development given [Syrian President Bashar] Assad's blatant disregard for commitments to Mr. Annan's peace plan," said Baird in a statement, adding that the six-point plan is now effectively dead.
Baird called on UN member states to support a resolution — which goes before the General Assembly on Friday -- condemning helicopter attacks and heavy arms assaults by government troops against rebel fighters, and for the Security Council to impose economic sanctions and an arms embargo on Syria.
Last-minute edits 'to improve the vote count'
On Thursday, the Arab nations made a last-minute revision to the resolution, removing a section of text that demanded that Assad immediately relinquish power to a transitional government.
Also removed was a line calling for more sanctions against Syria.
CBC's Melissa Kent, reporting from the UN in New York, said the last-minute edits before Friday's anticipated vote were an apparent bid to win more support among the 193 members of the General Assembly.
"Those lines were basically taken out to improve the vote count because they had stated other countries, such as maybe Pakistan, Brazil, might have abstained, and the Arab League wants a strong condemnation of Syria," Kent said. "So they want strong support for their proposition in Syria."
The vote on Friday would be a symbolic move. Diplomats have said it would be a moral statement meant to express mounting anger with the more authoritative Security Council's failure to reach an international agreement on how to stop Assad.
U.S. data-sharing support
The assembly has no legal mechanism for actually enforcing a resolution.
Pressure to oust Assad is building as deadly fighting escalates in Aleppo, Syria's most populous city.
Meanwhile, U.S. media is reporting that President Barack Obama signed a "secret order" earlier in the year to support the rebels in Syria by giving American agents permission to provide intelligence that would help the opposition on the ground.
CBC's Lyndsay Duncombe, reporting from Washington, said Thursday that while the order would not entail the provision of weapons to Syrian rebels, CIA staffers working out of a secret base in Turkey could still play a key information-sharing role.
As an example, Duncombe explained, the Americans could ensure that weapons supplied to opposition fighters from Saudi Arabia and Qatar end up in the right hands.
"There is some concern that among those rebel groups, there may be people who you don't want to give weapons to, such as al-Qaeda," she said.
"What the CIA could be doing with something like this order would be helping those other countries vet the people within the rebels to figure out who are the right folks that you want to be arming as you go into this fight."
Duncombe noted that the U.S. has already earmarked $25 million to assist Syrian rebels by providing non-lethal supplies such as communications equipment.
But the White House also made a rare rebuke of rebel tactics on Thursday, responding to video footage of prisoners being executed by opposition forces.
The video, which surfaced online this week, is fueling concerns that opposition fighters are capable of brutality that matches that of the regime they are seeking to topple.
"This is abhorrent and inconsistent with the type of struggle for freedom and a new Syria that the broad opposition is looking for," said State Department spokesman Patrick Ventrell.
Ventrell said the U.S. was encouraged that opposition commanders have condemned abuses, but stressed that "summary executions committed by any party are abhorrent and inconsistent with international law, and those responsible must be held to account."
Separately, Ventrell also criticized what he described as another "massacre" by Assad's forces, this time in the Damascus suburb of Yalda.