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.
With international diplomacy virtually paralysed by big-power splits over how to tackle the Syrian crisis, the clashes in Damascus seem to have carried the conflict into a new phase.
The fighting in the capital may have started on Sunday as a case of security forces tackling armed rebels displaced by a crackdown on the suburbs.
But the Free Syrian Army has now declared that Operation Damascus Volcano is under way.
The Muslim Brotherhood, the biggest and most organised opposition group, has called it a "decisive battle" and urged all Syrians to join a nationwide civil insurrection.
The regime is clearly discomfited.
With parts of the capital's centre paralysed, and thousands of residents displaced within the city limits, state media have dropped all mention of the Damascus fighting.
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.
Meanwhile, 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.