Damascus was tense on Tuesday as clashes between the Syrian army and rebels near the city center extended into a third day, with government forces throwing a security cordon around some embattled neighborhoods, firing from helicopters and reinforcing the number of tanks on the streets.

The urban combat in Damascus overshadowed international diplomacy aimed at halting the Syria conflict, which intensified ahead of a United Nations Security Council vote this week on whether to extend the mission of 300 United Nations monitors, who have been basically trapped in their hotel rooms since last month, their work suspended.

There was also new evidence, reported by Israel’s intelligence chief, that President Bashar al-Assad was moving troops into Damascus from Syria’s border with the disputed Golan Heights territory held by Israel, a possible indication of the seriousness of the fighting that was roiling neighborhoods at the president’s doorstep.