BEIRUT, Lebanon — Fierce fighting on the battlefield and setbacks on the diplomatic front increased pressure on the embattled Syrian government on Monday as fresh signs emerged of a worsening battle for control of the capital.

A senior Turkish official said that Russia had agreed on Monday to a new diplomatic approach that would seek ways to persuade President Bashar al-Assad to relinquish power, a possible weakening in Russia’s steadfast support for the government. 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 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 were grave concerns in United States 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.”