He also asserted that Syrian President Bashar Assad must leave office for the good of his country.
Annan told reporters that when he accepted the job, "which some called 'Mission Impossible,"' he wanted to help the international community, led by the U.N. Security Council, find a peaceful solution to the crisis. The goal was to stop the killings of civilians and human rights abuses, as well as to place Syria on a path toward political transition.
"The severity of the humanitarian costs of the conflict, and the exceptional threats posed by this crisis to international peace and security, justified the attempts to secure a peaceful transition to a political settlement, however daunting the challenge," Annan said.
But the former U.N. secretary-general told reporters that he cannot go on when the New York-based, 15-nation Security Council doesn't fully back him, particularly because of the stalemate between its five veto-wielding members: Russia and China on one side, the United States, Britain and France on the other.
"Things fell apart in New York," he summed up. "The increasing militarization on the ground (in Syria) and the clear lack of unity in the Security Council have fundamentally changed the circumstances for the effective exercise of my role."
Annan was named the U.N.-Arab League envoy to Syria in February, overseeing a small staff in a secretive office accessible through a makeshift elevator entrance in the sprawling Palais des Nations, the U.N.'s European headquarters in Geneva. He came up with a six-point peace plan to resolve Syria's crisis, including a cease-fire that was supposed to take effect in mid-April.
But, despite the presence of hundreds of U.N. observers on the ground, the cease-fire never took hold and the violence in Syria has morphed into a civil war. Rights activists say that more than 19,000 people have died since the popular uprising against Assad began in March 2011.
Annan notified U.N. Secretary-General Ban Ki-moon and the Secretary-General of the League of Arab States, Nabil El Araby, that he would leave office when his appointment expires Aug. 31.
"The bloodshed continues, most of all because of the Syrian government's intransigence, and continuing refusal to implement the six-point plan, and also because of the escalating military campaign of the opposition -- all of which is compounded by the disunity of the international community," Annan said.
"At a time when we need -- when the Syrian people desperately need action -- there continues to be finger-pointing and name-calling in the Security Council."
On June 30, Annan succeeded in getting the major powers on the council -- including stalwart Syria allies Russia and China -- to agree on a broad framework for a political transition in Syria, one that he said "meant President Assad would have to leave sooner or later." But the Security Council never formally endorsed the plan or acted on it, something that sorely disappointed the envoy and, he said, undermined his efforts.
Without international unity, including the cooperation of regional powers, he concluded: "it is impossible for me, or anyone, to compel the Syrian government in the first place, and also the opposition, to take the steps necessary to begin a political process. You have to understand: as an envoy, I can't want peace more than the protagonists, more than the Security Council or the international community for that matter," he told reporters at an impromptu press conference in Geneva.
Annan did not single out any member of the Security Council by name for criticism during the press conference. But in an op-ed column published by the Financial Times on Thursday, he urged several countries to look beyond their national interests and rivalries to solve the Syrian crisis.
"For Russia, China and Iran this means they must take concerted efforts to persuade Syria's leadership to change course and embrace a political transition, realizing the current government has lost all legitimacy," Annan wrote, adding: "For the U.S., U.K., France, Turkey, Saudi Arabia and Qatar this means pressing the opposition to embrace a fully inclusive political process -- that will include communities and institutions currently associated with the government."
Annan also had harsh words for the Syrian government and what he called its "intransigence." In what was his strongest statement to date about the Syrian leader, Annan wrote, "It is clear that President Bashar al-Assad must leave office."
Annan's announcement coincided with Arab countries dropping a demand that Assad resign in the latest draft of a symbolic U.N. General Assembly resolution that faces a Friday vote in New York. The watered-down resolution further illustrated the international struggle to build an effective diplomatic approach to Syria's civil war.
The draft resolution was resisted by countries such as Brazil, Russia, India, China and South Africa who had problems with calling for regime change or sanctions. Russia and China have vetoed stronger proposals on Syria in the Security Council, which, as the most powerful arm of the U.N., can adopt enforceable resolutions and impose sanctions.
In the U.S., White House press secretary Jay Carney said Annan's resignation highlighted the failure of Russia and China to support action against Assad. In Britain, Prime Minister David Cameron said Annan's departure was evidence that the current approach has failed -- and that the U.N. needs to get tougher with Syria.
"We've got this appalling bloodshed. I think what we need to do is to ramp things up," Cameron told Sky News television.
Ban, the U.N. secretary-general, said he is discussing possible successors with the Arab League. "I remain convinced that yet more bloodshed is not the answer; each day of it will only make the solution more difficult while bringing deeper suffering to the country and greater peril to the region," Ban said.
Russian President Vladimir Putin called the events in Syria a "tragedy."
"Kofi Annan is an honorable man and a brilliant diplomat, so I regret that very much," he said. "But I hope that the international community will continue efforts to end violence."
Alexei Pushkov, the head of the foreign affairs committee in the lower house of the Russian parliament, blamed the failure of Annan's plan on the West.
"Regrettably, the so-called Friends of Syria led by the United States have encouraged the opposition and sought to put pressure only on the Syrian government. That became the reason behind the failure of Annan's plan," Pushkov said, according to RIA Novosti.
China's Foreign Ministry expressed "regret" on Annan's leaving, saying in a statement on its web site, "We understand the difficulties in Annan's mediation work and respect his decision."
It is unclear how Annan's departure will affect his legacy.
Annan, the grandson of Ghanaian tribal chiefs, joined the United Nations in 1962 as an administrative and budget officer for the World Health Organization in Geneva.
He rose to become its secretary-general from 1997 to 2006, helping to ease the transition to civilian rule in Nigeria in 1998 and visiting Iraq to defuse an impasse between Saddam Hussein and the U.N. In 1999, he was deeply involved in helping East Timor gain independence from Indonesia.
But he also has been strongly associated with the criticism over some of the U.N.'s biggest failures.
He headed U.N. peacekeeping at a time when the U.N. has been accused of ignoring evidence that genocide was planned in Rwanda and of abandoning the Rwandan people when it was under way in 1994. The U.N. was also blamed for failing to help save thousands of Bosnian Muslims from the 1995 Serb mass murder in the town of Srebrenica, which had been declared a U.N. safe haven.
Asked Thursday about the idea of someone else being appointed to succeed him as envoy to Syria, Annan said, "The world is full of crazy people like me, so don't be surprised if someone else decides to take it on."
Associated Press Vladimir Isachenkov in Moscow and Peter Spielmann in New York contributed to this report.
More Nation & World Headlines
Police say person of interest in custody in Memphis officer killing 12:35 p.m. Obama to require steeper emissions cuts from U.S. power plants 12:56 p.m. Zimbabwe alleges 2nd American illegally hunted lion 12:11 p.m. Who will be the new face of the GOP? Malaysia seeks help in finding more possible MH370 debris 12:28 p.m. New bug-inspired robot can jump off of water Hitchhiking robot destroyed in Philadelphia U.S., Russia ease travel rules for Bering Strait natives
Our new comment system is not supported in IE 7. Please upgrade your browser here.