Fighting is intensifying across the Syrian capital Damascus, where the military has deployed tanks and helicopters, activists say.
Shooting was reported in one of the main central streets and a square housing the Central Bank.
Rebel forces say they have launched an all-out assault on the capital, calling it Operation Damascus Volcano.
UN envoy Kofi Annan has held talks with Russian President Vladimir Putin in Moscow over the crisis.
Mr Putin told reporters after meeting Mr Annan that Russia still backed the envoy's six-point peace plan, which called for a ceasefire and talks between the government and opposition.
Russia and China have consistently refused to back any measures that hint at intervention in Syria, or ask for Mr Assad to step down.
The UN has until Friday to renew the mandate for observers in Syria, and Western nations want the two nations to back tougher measures to stop the fighting.
UN Secretary-General Ban Ki-moon has landed in Beijing, where he called for rapid unified action by the Security Council to tackle the crisis.
In other developments:
Defector Nawaf al-Fares tells the BBC the regime has colluded with al-Qaeda in bomb attacks, and would not hesitate to use chemical weapons if it is cornered
Iraq warns its citizens to flee the violence, hours after the bodies of two killed journalists were handed over by the Syrian authorities
Activist video on Tuesday showed government tanks and troops moving on the main roads into the centre of Damascus.
The rebel Free Syrian Army said it had launched Operation Damascus Volcano, and has called for an escalation of attacks on regime targets and the blocking of main roads all around the country.
One of the biggest and most organised opposition groups, the Muslim Brotherhood, has called on all Syrians to join what it called a decisive battle.
Witnesses say the government's military deployment in Damascus is the biggest since protests against President Bashar al-Assad's rule began in March last year.
Clashes were reported in a major thoroughfare, Baghdad Street, the first time fighting has reached central Damascus since the conflict began.
Also, machine-gun fire was reported in nearby Sabaa Bahrat square, site of the Central Bank and scene of several major pro-government demonstrations.
Activists reported continued clashes on the south-western side of the city, including in Midan.
"The army is shelling al-Midan hysterically; the collapsing regime has gone mad," one activist told AFP.
Fighting is also said to have broken out on the other side, at Barzeh and Qaboun. Attack helicopters were seen there firing rockets for the first time since the uprising began.
Rebels told Reuters news agency they had shot down a helicopter in Qaboun.
Syrian Information Minister Omran Zoabi told Reuters that "some armed elements had infiltrated Damascus".
"The security forces surrounded them and dealt with them - and are still dealing with them," he said.
"Some [fighters] have surrendered and others escaped on foot and by car and are firing randomly in the air to frighten people," he said.
The BBC's Jim Muir in neighbouring Lebanon says it appears the uprising has moved into a new phase in the heart of the capital, paralysing parts of it for a time and causing panic.
Meanwhile, the head of Israeli army intelligence said Syrian forces had been redeployed from the border with the Israeli-occupied Golan Heights to help defend the capital.
"The Syrian military is acting very brutally, which shows the regime is desperate. Its control of Damascus is getting weaker," Maj-Gen Aviv Kochavi told a parliamentary committee, according to a Knesset spokesman.
Mr Fares, the former Syrian ambassador to Iraq, is the most senior Syrian politician to have defected to the opposition.
Syria is known to have a stockpile of chemical weapons and there are growing concerns about the security of such weapons should the government fall.
Asked if he thought President Assad might use chemical weapons against the opposition, Mr Fares told BBC security correspondent Frank Gardner that he would not rule it out, describing Mr Assad as "a wounded wolf".