Kofi Annan, the U.N.-Arab League envoy for Syria, said Thursday that he will step down from his post at the end of the month, departing with a stinging criticism of the international community and warring parties for not stopping the violence in the war-torn country.

Annan’s resignation deals a decisive blow to the already faltering international efforts to find a diplomatic solution to a crisis that has left more than 14,000 dead and pitched the country into deepening civil war.

The announcement came as Syria braced for a major armed clash between the government and rebels in the contested city of Aleppo, U.N. officials said. “We have reason to believe the main battle is about to start,” the United Nations’ peacekeeping chief told reporters after briefing the U.N. Security Council on Thursday.

At an impromptu news conference in Geneva, Annan said the international community and Security Council had not supported his efforts to enforce a cease-fire and bring about a transition of power.

“When the Syrian people desperately need action, there continues to be finger-pointing and name-calling in the Security Council,” the former U.N. secretary general said, according to a transcript. “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’s sudden resignation was announced in New York by Ban Ki-moon, the U.N. secretary general. He said that he had accepted Annan’s request to leave with “deep regret” and that he will mount a search for a replacement.

In announcing Annan’s resignation, the U.N. chief faulted both the government of President Bashar al-Assad and the Syrian opposition for refusing to embrace Annan’s six-point peace plan. He also accused the Security Council of not providing Annan with the political backing he needed to succeed.

“Tragically, the spiral of violence in Syria is continuing,” Ban said in a statement released from U.N. headquarters. “The hand extended to turn away from violence in favor of dialogue and diplomacy — as spelled out in the six-point plan — has not been not taken, even though it still remains the best hope for the people of Syria.”

Secretary of State Hillary Rodham Clinton, who called Annan Thursday to thank him for his service, said in a statement that he “worked tirelessly to try to build consensus” to end bloodshed in Syria, but “the Security Council was blocked from giving him key tools to advance his efforts.”

With the Security Council deadlocked over Syria, the U.N. General Assembly is expected to vote Friday morning on a resolution denouncing the Assad regime for unleashing heavy-weapon attacks on the cities of Damascus and Aleppo.

On Thursday morning, Syrian rebels attacked a military air base about 20 miles north of Aleppo with captured tanks, perhaps the first time rebels have attempted a sophisticated assault with heavy weaponry and a clear sign that the battle between opposition fighters and the government is escalating.

The attack began when rebel forces used three tanks to blast the Menagh air base from a distance. “There was confusion in the airport and several wounded,” said Col. Malik Kurdi, the deputy commander of the Free Syrian Army. “But the operation did not achieve the goals we planned and we were not able to occupy the airport.”

The Menagh airport is the base for the helicopters that are used in daily attacks in Aleppo and Idlib. The rebels want to take over the air base so they can safely send more fighters into Aleppo, Kurdi said. “Controlling this airport would protect the back of the Free Syrian Army when we attack and progress to the city of Aleppo,” he said.

Despite the mixed results in Thursday’s attack, Kurdi said it will not be the last time rebel forces use tanks, which are being operated by soldiers who defected from the military’s tank divisions. The rebels have captured 10 tanks in the past 20 days, Kurdi said.

Rebels have boasted of a number of successes in their fight for Aleppo, taking over five police stations and holding territory within the city, which the group was not able to do in Damascus.

There are signs that the stepped-up rebel assault is taking a toll. Syrian state TV has begun airing recruitment ads for policemen, a rarity in recent years. Entry requirements for applicants have been dropped: a sixth-grade education is the minimum necessary, whereas applicants in the past had to have completed high school.

Still, the Syrian government has hardly given up the fight. Artillery shells battered the Salahuddin neighborhood in Aleppo as jets fired on rebel positions Thursday, according to opposition groups. Heavy shelling also was reported in the Damascus neighborhood of Tadamon, which has been relatively quiet in the past week as government forces have tried to reassert control.

At least 130 people were killed in Syria on Thursday, including about 50 in and around Damascus and seven in Aleppo, according to the Local Coordination Committees, an activist network.

Annan’s departure darkens the picture for a diplomatic solution. His resignation came nearly two weeks after Russia and China blocked a Western-backed resolution that would have reinforced his mediation effort with the threat of sanctions. It was their third veto since fall.

Vitaly I. Churkin, Russia’s U.N. envoy, defended his government’s position, saying that Moscow has made a greater commitment to Annan’s peace plan than many of Washington’s allies.

The impasse in the Security Council undermined Annan’s negotiating leverage and set the stage for the Syrian forces to enter a more violent phase of the civil war, council diplomats said.

In the past two weeks, the United States and its Western partners have been looking for ways to increase their support for the armed opposition, but they have stopped short of committing lethal support to the anti-Assad movement, fearing it could potentially benefit foreign extremists, including al-Qaeda, seeking inroads in Syria.

Said Ban: “The government and the opposition forces continue to demonstrate their determination to rely on ever-increasing violence. In addition, the persistent divisions within the Security Council have themselves become an obstacle to diplomacy, making the work of any mediator vastly more difficult.”

It is unclear who would be willing to replace Annan. The United Nations had previously considered asking former Finnish president Martti Ahtisaari to fill the position.

Ban said he is “indebted” to Annan for taking on such an intractable diplomatic challenge, saying: “Kofi Annan deserves our profound admiration for the selfless way in which he has put his formidable skills and prestige to this most difficult and potentially thankless of assignments.”

He said the United Nations, which continues to oversee a monitoring mission in Syria, is committed to pursuing a diplomatic outcome. “This can only succeed — indeed any peacemaking effort can only prosper — when the parties to the violence make a firm commitment to dialogue, and when the international community is strongly united in support,” he said.

Babak Dehghanpisheh, Suzan Haidamous, Anne Gearan and Ahmed Ramadan contributed to this report.