The fighting over the past few weeks in and around Damascus has been the most serious in the capital since July, when rebels captured several neighborhoods before a swift government counteroffensive swept them out. The spike in violence recently is concentrated in the ring of mostly poor suburbs around Damascus but often bleeds into the capital itself as rebels bring their fight closer to Assad's seat of power. Assad's forces have so far repelled major rebel advances on the capital, though their hold may be slipping.
"The security situation has become extremely difficult, including in Damascus," said Radhouane Nouicer, the U.N.'s regional humanitarian coordinator for Syria.
Nouicer said the U.N. was withdrawing most of its international staff from Syria due to security issues, adding that up to one quarter of the 100 international staff working for several U.N. agencies could leave by week's end. There are about 900 more local staff working for the U.N. in Syria, officials said.
U.N. teams are also stopping most staff trips outside Damascus.
In another sign of deteriorating security, an Egyptian commercial jet aborted a trip to Damascus in mid-flight because of violence near the airport. The EgyptAir flight from Cairo rerouted about 30 minutes after takeoff because Egyptian officials received word from their counterparts in Damascus that the area near the airport was not safe, Egyptian airport officials said.
EgyptAir canceled all further flights to Syria for Monday and Tuesday and will decide later whether to resume flights later in the week, the officials said, speaking on condition of anonymity because they were not authorized to speak to reporters.
EgyptAir had just resumed flights following a three-day suspension because of violence near the airport.
Emirates airlines said on its website that all flights to Syria were suspended "until further notice."
The Britain-based opposition activist group Syrian Observatory for Human Rights said the clashes were within t2 miles of the airport, which lies about 15 miles southeast of the city center. The state news service reported clashes in an area about 9 miles from the airport. It said nothing about flight cancelations.
More than 40,000 people have been killed since the Syrian revolt started in March last year.
Leaders of Russia — a key supporter of the Assad regime — and opposition ally Turkey discussed disagreements over Syria.
After talks in Istanbul, Russian President Vladimir Putin said that the deployment of NATO antiaircraft missiles along the Turkish border could make the conflict worse.
"Creating a new potential on the border will not settle the situation but rather exacerbate it," he said. "Why would we need more shelling on the border?"
The two countries are firmly enmeshed in Syria's conflict, on opposite sides. Russia continues to back Assad, thrice protecting his regime from censure by the U.N. Security Council. Turkey has called for Assad's ouster and its southern border with Turkey has become a key supply line for rebel forces.
Lebanese security officials said Makdissi, the Foreign Ministry spokesman known for defending Assad crackdown on the opposition as necessary military action against "terrorists," flew from Beirut to London. It was not immediately known whether he had abandoned the regime and he did not respond to phone calls.
The officials spoke on condition of anonymity because they were not authorized to brief the media.
The U.S. has so far declined to intervene in Syria's civil war, saying doing so could worsen the conflict.
On Monday, Secretary of State Hillary Rodham Clinton reiterated the Obama administration's position that Syrian use of chemical weapons was a "red line" whose crossing would prompt U.S. action.
"I'm not going to telegraph in any specifics what we would do in the event of credible evidence that the Assad regime has resorted to using chemical weapons against their own people," Clinton told reporters in Prague. "But suffice it to say, we are certainly planning to take action if that eventuality were to occur."
Syria is believed to have several hundred ballistic surface-to-surface missiles capable of carrying chemical warheads, and a U.S. defense official said American and allied intelligence officials have detected activity around more than one of Syria's chemical weapons sites in the last week.
He spoke on condition of anonymity because he was not authorized to speak to reporters about intelligence matters.
Syria's Foreign Ministry said Monday that Syria "will not use chemical weapons — if there are any — against its own people under any circumstances."