Iraq has closed its main border post with Syria and sent troops to the frontier after Syrian rebels took control of the Abu Kamal crossing, a local mayor has said.
"Qaim post, opposite Abu Kamal, is closed and now under the control of the Iraqi army," the mayor of Qaim, Fahan Fitaihan, said.
Iraqi troops had also taken over another nearby post from Iraqi customs officials, he said.
He said the Syrian Free Army had raised its flag on the other side of the border.
Earlier, Syrian rebels battled deep into the heart of Damascus against government troops, who unleashed artillery and helicopter gunships.
Diplomatic efforts collapsed when Russia and China vetoed a UN Security Council resolution that would have imposed sanctions on Syrian authorities unless they stop using heavy weapons.
Washington said the Council had "failed utterly".
President Assad's brother-in-law, his defence minister and a top general were killed in yesterday's bomb attack on a crisis meeting inside a security headquarters.
Mr Assad's own failure to appear in public in the next 24 hours - he was shown today in television pictures of the swearing in of a replacement for his slain defence minister - compounded the sense of his power eroding.
Residents said a heavy onslaught of security force shelling and firing from helicopters went on through the night and continued today in Damascus.
Some reported explosions in the capital's troubled northeastern and southern districts.
Other parts of the city were paralysed. Homes and shops were shuttered as Damascenes feared violence.
Lebanese officials said refugees were pouring across the frontier. A security source said 20,000 Syrians had crossed.
Reports from Syria cannot be independently verified due to state restrictions on foreign media.
The failed UN Security Council resolution, which would have extended a small, unarmed UN monitoring mission, was the third that has been vetoed by Russia and China.
US ambassador to the United Nations Susan Rice said the Security Council had "failed utterly", and Washington would look outside the body for ways "to bring pressure to bear on the Assad regime and to deliver assistance to those in need".
Britain proposed a new four-paragraph resolution that would extend the mission for another 30 days, that could be voted on later, diplomats said.
While fighting rages in Damascus, clashes and shelling have also continued elsewhere across the country.
Rebels said they had "liberated" the town of Azaz in northern Aleppo province, bordering Turkey. Activists also published video of the town of Talbiseh, in central Homs province, being sprayed with gunfire from helicopters above.
Fighting also erupted near the Syrian-Israeli frontier, and Israel promptly responded saying it would not accept refugees.
"In the event of the regime's downfall, which could happen... (Israeli forces) here are alert and ready, and if we have to stop waves of refugees, we will stop them," Defence Minister Ehud Barak said.
