BEIRUT — A senior Turkish official said that Russia had agreed Monday to a new diplomatic approach that would seek ways to persuade President Bashar Assad to relinquish power, a possible weakening in Russia’s steadfast support for the government.

Meanwhile, fighting raged around Damascus, the Syrian capital, and its airport, disrupting commercial flights for a fourth straight day.

A prominent Foreign Ministry spokesman was said to have left the country amid reports of his defection, and both President Barack Obama and Secretary of State Hillary Rodham Clinton issued warnings that any use of chemical weapons by a desperate government would be met with a strong international response. A Western diplomat confirmed that there are grave concerns in U.S. intelligence circles that Syrian leaders could resort to the use of the weapons as their position deteriorates.

The Syrian Foreign Ministry, repeating earlier statements, told state television that the government “would not use chemical weapons, if it had them, against its own people under any circumstances.”

The United Nations said it was withdrawing nonessential international staff from Syria, and the European Union said it was reducing activities in Damascus “to a minimum,” as security forces pummeled the suburbs with artillery and airstrikes in a struggle to seal off the city from its restive outskirts and control the airport road. A senior Russian official spoke for the first time in detail about the possibility of evacuating Russian citizens.

Assad has held on longer than many had predicted at the start of the 21-month uprising. He still has a strong military advantage and undiminished support from his closest ally, Iran.

Military analysts doubt the rebels are capable of taking Damascus by force, and one fighter said Monday that the government counteroffensive was inflicting heavy losses.

A Russian political analyst with contacts at the Foreign Ministry said that “people sent by the Russian leadership” who had contact with Assad two weeks ago described a man who has lost all hope of victory or escape.

“His mood is that he will be killed anyway,” Fyodor Luk-yanov, editor of a Russian foreign affairs journal said.

After meeting in Istanbul on Monday, Russian President Vladimir Putin and President Recep Tayyip Erdogan of Turkey said they had agreed on a new approach to resolving the conflict.

Mikhail Bogdanov, a deputy foreign minister, said Russia would meet intensively with Syrian opposition groups based inside the country in the coming month. A senior Turkish official, speaking anonymously in accordance with diplomatic protocol, said plans included looking for ways to get Assad to step down.