BBC News reports: Armoured personnel carriers have been deployed in areas of Damascus, Syrian activists say, with clashes spreading on a second day of fighting.
The activists said troops backed by armoured vehicles had entered the Midan district to try to dislodge rebels.
Witnesses say this appears to be the biggest military deployment in the capital in the 16-month uprising.
Meanwhile, Russia said Western attempts to get Moscow to discuss sanctions contained “elements of blackmail”.
The two days of clashes appeared to be the heaviest fighting in Damascus in the uprising against the rule of President Bashar al-Assad, activists said.
The BBC’s Jim Muir says it is not clear whether the battle for Damascus has begun, but the violence seems to be creeping ever closer to the heart of the capital and the centre of the government’s power.
The armoured troop carriers took up positions on the main roads in Midan, which is a mainly Sunni district.
The Syrian Observatory for Human Rights said armour had not previously been deployed in Midan.
Its director, Rami Abdel Rahman, told Agence France-Presse: “Before, the security forces were deployed to suppress protests. Now, we have army troops engaged in combat.”
He added: “[It has] never been this intense.”
One resident, who lives in the south of the city, told the BBC’s Newshour there was a lot of tension, with people scared and nervous, and it was becoming difficult to travel.
He said: “It’s mainly in the southern parts of the city which are effectively besieged at the moment. There were very few people on the streets, just totally different from how the city is normally.
“The feeling, among people around me, is that it’s our turn now. We are really feeling this. That this is the final fight, building up to who wins control of the regime.”