The developments were reported a day after the expiration of a four-day truce for the Muslim holiday of Id al-Adha that had been widely and persistently violated. Each side accused the other of subverting the cease-fire, which was negotiated by the special Syria peace envoy of the United Nations and the Arab League, Lakhdar Brahimi. Mr. Brahimi had said he hoped it would form the basis for the beginning of dialogue between President Bashar al-Assad's loyalists and his armed opponents.

An announcement carried on state television said the air force commander, Gen. Abdullah Mahmud al-Khalidi, was killed in the Damascus district of Rukn al-Din on Monday by armed terrorist groups, the government's categorical term for its adversaries.

The announcement did not specify how the commander had been killed, but it described him as one of the country's top aviation experts. Agence France-Presse, in a report from Damascus, quoted an unidentified security source as saying the commander had been shot to death while leaving a friend's home.

The news agency also said that the Free Syrian Army, the main group of armed insurgents in Syria, had claimed responsibility for the assassination. But other unconfirmed reports from Syria raised the possibility the general had been killed by government agents to prevent him from defecting. Al Jazeera quoted unidentified activists as saying "the regime got rid of him before he does that."

Syrian insurgents have made no secret of their intent to kill Mr. Assad's top aides. In July, insurgent bombers killed the defense minister, deputy defense minister and assistant vice president as they met in a Damascus safe house, the biggest single blow to Mr. Assad's inner circle since the uprising against him began in March 2011.

Reports from opposition activists, including the Syrian Observatory for Human Rights, a group based in Britain with a network of contacts in Syria, said the official end of the cease-fire on Monday brought a large expansion of aerial bombardments on Tuesday, including a sortie by a fighter jet that dropped four bombs on the Damascus neighborhood of Jobar. The observatory said it was the first reported instance of a fighter jet attack in Damascus airspace, where the military had previously used helicopter gunships.

The Syrian Observatory said at least 23 people were killed Tuesday, most of them in Syrian government airstrikes in the contested town of Douma, near Damascus. There was no way to corroborate those casualty figures.

There were also unconfirmed reports of clashes between rebels and pro-government Palestinian fighters near the large Yarmouk Palestinian refugee camp in the southern Damascus area. The camp is the largest community of Palestinian refugees in Syria, and the loyalty of its residents is politically important to Mr. Assad, who considers himself a critical ally of militant Palestinians and has often characterized the uprising against him as part of an Israeli plot.

Mr. Brahimi, an Algerian statesman who replaced Kofi Annan, the former United Nations secretary general, as the special Syria envoy nearly two months ago, has expressed regret that his cease-fire plan failed. But he has said he intends to present a broader plan to the United Nations Security Council in November.

He has spent the past few days in visits to Russia and China, the two permanent members of the Security Council that have blocked efforts by the other members to approve any resolution that would threaten Mr. Assad with punitive consequences.