Kofi Annan announced his resignation Thursday as peace envoy to Syria and issued a blistering critique of world powers, bringing to a dramatic end a frustrating six-month effort that failed to achieve even a temporary cease-fire as the country plunged into civil war.

Annan also had harsh words for the Syrian regime, saying it was clear President Bashar Assad “must leave office.”

As the violence escalated on the ground, rebels used a captured tank to shell a military air base near Aleppo _ one of the first known uses of heavy weapons by the insurgents.

Speaking to reporters in Geneva, Annan blamed the Syrian government's intransigence, the growing militancy of Syrian rebels and a divided Security Council that failed to forcefully back his effort. Since he took on the job, Russia and China have twice used their veto power to block strong Western- and Arab-backed action against President Bashar Assad's regime.

The White House said Annan's resignation highlighted the failure of Russia and China to support action against Assad and called the regime's continued violence against its own people “disgusting.”

“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,” said Annan, a Nobel Peace Prize laureate and former U.N. secretary general.

“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.”

Annan singled out the regime for blame for the violence. But he also said the opposition's increasing militarization had contributed to dooming his six-point peace plan, which included a cease-fire and a Syrian-led political process to end the crisis.

“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,” he 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.”

U.N. Secretary-General Ban Ki-moon said he accepted the resignation with deep regret, adding that the search was under way for a successor to Annan, who will stay on until Aug. 31. Diplomacy can succeed only when “the parties to the violence make a firm commitment to dialogue, and when the international community is strongly united in support,” Ban said in a statement.

In an opinion piece published by the Financial Times on Thursday under the headline “My Departing Advice on How to Save Syria,” Annan had harsh words for all parties in the conflict. But he appeared to reserve particular criticism for the Assad regime, asserting in his strongest statement to date about the Syrian leader: “It is clear that President Bashar al-Assad must leave office.”

During his six-month tenure, Annan managed, at least in theory, to get world powers behind his plan _ including the Syrian government and its allies in Iran, Russia and China _ although his appeals for peace were ignored on the ground.

The regime and the rebels blamed each other for the violence, and Russia and China said attempts to sanction the regime ignored violations by the opposition.

Secretary of State Hillary Rodham Clinton praised Annan for working “tirelessly” to try to end the bloodshed in Syria. “Unfortunately, the Security Council was blocked from giving him key tools to advance his efforts,” she said.

Asked about Annan's resignation, the White House again called for Assad to step aside. The Syrian leader “continues to brutally murder his own people, to use heavy weapons in assaults on civilian population centers, to call on his military leaders to kill the Syrian people in his name,” White House press secretary Jay Carney said.

Russia's U.N. envoy Vitaly Churkin said Moscow regrets Annan's decision and Syria's Foreign Ministry also expressed “dismay.”

Alexei Pushkov, head of the foreign affairs committee in Russia's lower house of parliament, blamed 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,” he told the RIA Novosti news agency.

Amnesty International called Annan's departure the “culmination of a string of failures” and inaction by the U.N. Security Council that allowed the regime's “murderous campaign” to continue.

Annan was appointed envoy in February, representing the United Nations and the Arab League. But he was unable to calm the crisis, which began in March 2011 with mostly peaceful protests against the regime but has now morphed into a civil war.

According to activists, more than 19,000 people have been killed.

Even though Annan's plan appeared doomed from the start, world powers had few options to help beyond diplomacy _ in part because of fears that any military intervention could make matters worse. Syria's close ties to Iran and the Islamic militant group Hezbollah in Lebanon mean that the conflict has the potential to draw in the country's neighbors. 

As the battle for the country intensifies, the rebels used a tank they captured from Syrian soldiers to pound a military air base Thursday in the country's largest city, Aleppo _ an escalation that all but guarantees an even bloodier civil war.

Since opposition forces first started taking up arms against the regime, they have suffered from the huge disparity with Syria's well-armed military, which has tanks, fighter jets and helicopter gunships at its disposal.

It's not clear whether the rebels will be able to use tanks in a sustained way, as they require fuel and ammunition that is in short supply. But their ability to capture such weapons suggests growing coordination and sophistication.

Still, the Syrian army has far more powerful weapons than the rebels and there was no indication that Thursday's attack on the air base was particularly effective. Later, a nearby village was shelled by government forces out of that same air base.

There was also heavy shelling around the town of Azaz on the Turkish border, which has been in rebel hands for weeks along with a nearby checkpoint crossing, key to delivering rebel weapons and supplies to the Aleppo battle. It would be a huge blow to the opposition if the government retook the crossing.

Rebels attacked Aleppo two weeks ago and have captured several neighborhoods, holding out against bombardments by the government.

With its proximity to rebel-friendly Turkey to the north, Aleppo has enormous strategic importance to the opposition and if the rebels were able to capture and hold it, the city could form the basis for a wider rebel-controlled zone.

In the capital, Damascus, the regime announced a string of raids against rebels in neighborhoods on the southern edge of the city, killing and arresting a number of “terrorists” _ the government's term for its opponents.

Activists reported dozens killed, but that could not be independently confirmed.

Also on Thursday, the regime bombarded the capital's southern suburb of Tadamon with artillery and mortars, sending plumes of smoke into the sky. The regime crushed a bold rebel assault on Damascus two weeks ago, but the latest raids show that pockets of resistance remain in the capital and the surrounding countryside.

A vote was set for Friday in the U.N. General Assembly on a resolution drafted by Arab League countries. But its call for Assad to step down and for sanctions on Syria was dropped because of resistance from Russia, China and several other nations. While the 193-member General Assembly has no legal mechanism for enforcing a resolution, an overwhelming vote can carry moral and symbolic power. (AP)