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

A senior Turkish official said that Russia had agreed 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 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.”

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 pummelled 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 interviewed said the government counteroffensive was inflicting heavy losses.

There were still no firm indications from Russia that it was ready to join Turkey and western nations in insisting on Assad’s immediate departure.

But the latest grim developments follow a week of events that suggested the Assad government was being forced to fight harder to keep its grip on power. Rebels threatened its vital control of the skies, using surface-to-air missiles to down a fighter plane and other aircraft.

The opposition also gained control of strategic military bases and their arsenals, and forced the government to shut down the Damascus airport periodically. The Internet was off for two days.

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 Lukyanov, editor of a Russian foreign affairs journal and the head of an influential policy group, said in an interview in Moscow, adding that only an ‘extremely bold’ diplomatic proposal could possibly convince Assad that he could leave power and survive.

“If he will try to go, to leave, to exit, he will be killed by his own people,” Lukyanov said, speculating that security forces dominated by Assad’s minority Alawite sect would not let him depart and leave them to face revenge. “If he stays, he will be killed by his opponents. He is in a trap. It is not about Russia or anybody else. It is about his physical survival.”

Many observers — UN personnel in Syria, Arab diplomats and opposition activists — stress that it is difficult to reliably assess the state of the government. But taken together, the events suggested that the government’s position was declining more sharply than it had in months and that an international scramble to find a solution to the crisis was intensifying.

Powerful ally

The Arab League has long called for Assad to step down. But Russia, Assad’s most powerful ally, has held out the possibility of his staying in power during a transition, so the Russian government’s apparent shift of emphasis carried more weight.

After meeting in Istanbul on Monday, president Vladimir V Putin of Russia and prime minister Recep Tayyip Erdogan of Turkey said they had agreed on a new approach to resolving the conflict. “We are neither protecting the regime in Syria nor acting as their advocate, but remain worried about Syria’s future,” Putin said at a joint news conference with Erdogan.

“There is definitely a softening of the Russian political tone,” a Turkish official said, adding that Putin had acknowledged that Assad seemed unwilling to depart. Yet, doubts remain about whether Russia can engineer a breakthrough.

The Kremlin has insisted the crisis would be resolved only through negotiations between Syria’s government and its opponents, and its top envoy to Syria has quietly continued to meet with defectors from Assad’s government and members of the opposition.

But Russia has typically engaged mainly with Syria-based opposition groups, which the exile opposition and many in the uprising say are too close to the government. And Mr. Lukyanov, the Russian analyst, noted that even if Mr. Assad went, a radicalized Alawite security force could simply “turn into a militia.”

Lebanon’s Al-Manar television reported that a smooth-talking Syrian Foreign Ministry spokesman, Jihad Makdissi, had been fired for making statements that did not reflect the government’s position. Activists said he had defected.

Makdissi, whose polished persona and fluent English had long made him one of the most cosmopolitan faces of the government, had not taken reporters’ phone calls or made public statements recently.

Rami Abdul-Rahman, the director of the British-based Syrian Observatory for Human Rights, who uses a pseudonym for safety reasons, said that Mr. Makdissi had met his family in Beirut, where they had been staying, and was believed to have boarded a flight for London. He said Mr. Makdissi had earlier angered some in the Syrian government with a statement saying Syria would use chemical weapons only against a foreign invasion — weapons the government prefers not to acknowledge it has.

While the fighting around Damascus has been intense, analysts say rebels are probably unable to overrun the capital; rather, in forcing the government to devote forces to Damascus, their offensive could hasten the loss of control in other parts of the country.

“We feel a change in the security situation,” said Muhannad Hadi, the Syria director of the United Nations’ World Food Program. He played down the United Nations evacuations, saying that nonessential personnel had left during a rebel offensive in July and had returned. But he said that the proliferation of checkpoints and explosions in the distance had made life in Damascus nerve-racking.

“You hear sounds of explosions, you hear shelling, you don’t know where it’s taking off or where it’s landing,” Mr. Hadi said. “It’s becoming part of daily life.”