* Russia warns against unilateral U.S. action in Syria
* At least 20 bodies found after Syria troops overrun suburb
* Syria says foreign intervention "impossible"
By Dominic Evans
BEIRUT, Aug 21 (Reuters) - Russia warned the West on Tuesday against unilateral action on Syria, a day after U.S. President Barack Obama threatened "enormous consequences" if his Syrian counterpart used chemical or biological arms or even moved them in a menacing way.
Russian Foreign Minister Sergei Lavrov, speaking after meeting China's top diplomat, said Moscow and Beijing were committed to "the need to strictly adhere to the norms of international law...and not to allow their violation".
Syrian Deputy Prime Minister Qadri Jamil, also speaking in Moscow, dismissed Obama's threat as media fodder.
"Direct military intervention in Syria is impossible because whoever thinks about it ... is heading towards a confrontation wider than Syria's borders," he told a news conference.
Jamil said the West was seeking an excuse to intervene, likening the focus on Syria's chemical weapons with the run-up to the 2003 invasion of Iraq by U.S.-led forces on what proved to be groundless suspicions that Saddam Hussein was concealing weapons of mass destruction.
Russia and China have opposed military intervention in Syria throughout a 17-month-old revolt against President Bashar al-Assad. They have vetoed three U.N. Security Council resolutions backed by Western and Arab states that would have put more pressure on Damascus to end violence that has cost 18,000 lives.
In one of the latest battle zones, Syrian troops and tanks overran the Damascus suburbs of Mouadamiya on Tuesday, killing at least 20 young men and burning shops and houses before pulling back, residents and opposition activists said.
The bodies of the men, mostly shot at point-blank range, were found in basements and looted premises, bringing to 50 the death toll from the army's two-day-old offensive to drive rebels from the Sunni Muslim suburb in the southwest of the capital.
"People are just starting to get out of their homes to see the destruction," said one activist who gave her name as Hayat.
Opposition sources said Free Syrian Army rebels left Mouadamiya at dawn under heavy aerial and ground bombardment.
State-imposed curbs on media made it impossible to verify the reports of the violence, which followed another bloody day on Monday, when about 200 people were killed across the country, according to the opposition Syrian Observatory for Human Rights.
The United States and its allies have shown little appetite for intervention to halt the bloodshed on the lines of last year's NATO campaign that helped topple Libya's Muammar Gaddafi.
But Obama used some of his strongest language yet on Monday to warn Assad not to use unconventional weapons.
"We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is (if) we start seeing a whole bunch of chemical weapons moving around or being utilised," he said. "That would change my calculus."
Syria last month acknowledged for the first time that it had chemical or biological weapons and said it could use them if foreign countries attacked it.
"We cannot have a situation where chemical or biological weapons are falling into the hands of the wrong people," Obama said, perhaps referring to Lebanon's Shi'ite Hezbollah group, an Iranian-backed ally of Assad, or to Islamist militants.
The U.S.-based Global Security website says there are four suspected chemical weapons sites in Syria producing the nerve agents VX, sarin and tabun. It does not cite its sources.
Israel, still formally at war with Syria, has also debated whether to attack the unconventional arms sites which it views as its gravest peril from the conflict next door.
Obama has been reluctant to embroil the United States in another war in the Middle East and refuses to arm Syrian rebels, partly for fear that some of those fighting the Iranian-backed president are Islamist radicals equally hostile to the West.
Rebels have seized swathes of territory in northern Syria near Turkey, which now hosts 70,000 Syrian refugees and which has suggested that the United Nations might need to create a "safe zone" in Syria if that total topped 10,000.
But setting up a safe haven would require imposing a no-fly zone, an idea which U.S. Defense Secretary Leon Panetta said last week was not a "front-burner" issue for Washington.
With diplomatic efforts to end the war stymied by divisions between world powers and regional rivalries, Syria faces the prospect of a prolonged conflict that increasingly sets a mainly Sunni Muslim opposition against Assad's Alawite minority.
That sectarian faultline also flared in neighbouring Lebanon, where two people were killed and more than 60 wounded in the northern port city of Tripoli, a mainly Sunni city with a staunchly pro-Assad Alawite minority.
Gunmen in the Sunni district of Bab al-Tabbaneh and their Alawite rivals in Jebel Mohsen exchanged gun and grenade fire in sporadic fighting overnight and into the day, despite action by Lebanese army troops deployed in the port city, residents said.
The wounded included 10 soldiers, the army said.