* U.N. to pull out "non-essential" international staff
* Heavy bombardment, clashes in and around Damascus
* Clinton says U.S. will act if Syria uses chemical weapons
By Khaled Yacoub Oweis and Erika Solomon
CAIRO/BEIRUT, Dec 3 (Reuters) - A Syrian foreign ministry spokesman, who was the most public face of Bashar al-Assad's government as it battled a 20-month-old uprising, has fled the country, a diplomat in the region said on Monday.
Jihad al-Makdissi, who is in his 40s, previously worked at the Syrian embassy in London and returned to Damascus a year ago to serve as spokesman for the ministry, defending the government's crackdown on the revolt against Assad's rule.
He had little influence in a system largely run by the security apparatus and the military. But Assad's opponents will see the loss of such a high profile figure, if confirmed, as further evidence of a system crumbling from within.
Rebel forces have made advances in recent weeks, seizing military bases including some close to the capital Damascus. Amid talk that troops had moved chemical weapons, U.S. President Barack Obama again warned Assad against using them.
Makdissi belongs to Syria's Christian minority, which has largely stood behind Assad. He worked with the foreign ministry for 10 years and speaks fluent English, a rarity in a state apparatus shaped by the Baath Party's anti-Western ideology.
"He defected. All I can say is that he is out of Syria," the diplomatic source, who did not want to be named, told Reuters.
Lebanon's al-Manar Television, citing government sources, said Makdissi was sacked for making statements that did not reflect the government's position.
He was rarely seen in the media in recent weeks. His mobile telephone was switched off and there was no immediate comment in Syrian state media. The pan-Arab news channel Al Arabiya said Makdissi had left Beirut and was on his way to London, where he was expected to remain.
"We're aware of reports that he has defected and may be coming to the UK. We're seeking clarification," a Foreign Office spokeswoman said.
Rebels have begun to advance more quickly after months of slow sieges to cut off army routes and supplies. In the past few weeks, they seized several military bases, and are now using anti-aircraft weapons to attack the military helicopters and fighter jets that had bombarded their positions with impunity.
Media reports citing European and U.S. officials said Syria's chemical weapons had been moved and could be prepared for use in response - long a fear raised by the opposition.
White House spokesman Jay Carney said U.S. concerns about Syria's intentions regarding the use of chemical weapons were increasing, prompting Washington to make contingency plans.
Syria said it would not use chemical weapons against its own people: "Syria has stressed repeatedly that it will not use these types of weapons, if they were available, under any circumstances against its people," the foreign ministry said.
Obama, who has steered clear of repeating in Syria the kind of military engagements Washington has seen in Iraq, Afghanistan or Libya, later repeated a warning to Assad - vaguely worded - against using chemical weapons to keep himself in power:
"The world is watching," Obama said. "The use of chemical weapons is and would be totally unacceptable and if you make the tragic mistake of using these weapons there will be consequences and you will be held accountable."
The army appears to have focused most of its energy on Damascus, where rebels have been planning to push into the capital from the surrounding suburbs.
The military has been trying to seal off the city, using heavy bombardment and air raids to try to drive rebels back. Over 56 people were killed around Damascus alone on Sunday, with 200 dead across the country.
Damascus itself has not been free of unrest. Rebel-held southern districts have been bombarded heavily, activists say. The Syrian Observatory for Human Rights reported fierce clashes around the Tishreen military hospital in the northern Barzeh district and a car bomb in the southern area of Tadamon.
Neither side appears to have the upper hand in the fighting around Damascus. A previous attempt by rebels last July to hold ground in the city was crushed, but the fighters fell back into the suburbs and nearby countryside.
Clashes and tensions also remain high around Damascus International Airport and along the airport highway, which has become an on-and-off battleground that forced foreign airlines to suspend flights to Damascus since Thursday evening.
EgyptAir, which tried to resume flights on Monday, had to call back a plane headed to Damascus due to the "bad security situation" around the airport, an airline official said.
The conflict has grown increasingly bloody in recent months, as rebels began to contest Assad's power around the capital as well as in Aleppo, Syria's largest city. More than 40,000 people have died in the conflict, with hundreds more killed each week.
The United Nations said on Monday it was withdrawing "all non-essential international staff" from Syria because of deteriorating security, and was restricting remaining staff to Damascus. It said more armoured vehicles were needed following attacks on humanitarian aid convoys sometimes caught in the crossfire.
A European Union official said the EU was pulling international staff from Damascus because of the security situation and its ambassador to Syria had ended his posting.