There has been more fighting during the night around Syria's capital Damascus, activists say, days after rebels declared a final battle for the city.
Reports say that a barracks near the presidential palace is on fire.
However, Syrian officials dismissed claims of an all-out rebel offensive.
Meanwhile, UN chiefs have been trying to persuade China and Russia to agree tougher measures on Syria, ahead of a Security Council vote on Wednesday on imposing sanctions.
UN-Arab League envoy Kofi Annan held talks with Russian President Vladimir Putin in Moscow, and UN Secretary General Ban Ki-moon met Chinese leader Hu Jintao in Beijing.
The UN has until Friday to renew the mandate for observers in Syria. Western nations want a new resolution backed by the threat of force.
'Damascus volcano'Activists reported more clashes during the night in several areas around the south-west of Damascus.
They said the government had brought more troops and armour into some districts.
They have also posted on the internet pictures of what they say is an army barracks on the heights overlooking the city engulfed in flames.
They believed it had been hit by fire from Free Syrian Army (FSA) rebels, and said the barracks is involved in providing security for the presidential palace complex below.
State media said security forces fought off attacks by small groups of armed terrorists in the city.
But the TV carried night-time footage of troops deployed in the Midan quarter, in some very tense and deserted streets.
The rebels have declared a final battle for the capital, calling it Operation Damascus Volcano, and have been fighting troops in several parts of Damascus for the past three days.
The fighting reached central areas on Tuesday, with gunfire and plumes of smoke reported in a street near parliament.
The Free Syrian Army said the operation was well planned, and they had sent hundreds of fighters to the capital last week to be in place for the assault.
The rebels and the government often publish contradictory accounts of the same incidents.
Western journalists are under heavy restrictions in Syria, making it difficult to verify the claims of either side.
About 16,000 people have died in Syria since protests against the Assad regime began in March last year.
