Military helicopters are said to have pounded a rebel-held district of Aleppo and armoured units positioned themselves for an onslaught that could determine the fate of Syria's biggest city.
The Syrian Observatory for Human Rights reported helicopter attacks on the central Salaheddine district of Aleppo and fighting elsewhere in the city.
One opposition activist said he had seen Syrian tanks and armoured carriers heading for Salaheddine.
Reports from Syria cannot be independently verified due to state restrictions on foreign media.
The battle for the city of 2.5m people is seen as a crucial test for a government that has committed major military resources to retaining control of Aleppo and the capital Damascus.
However, neither side has so far managed to gain the upper hand.
The outcome of the uprising is being watched anxiously in the region and beyond.
There are fears that sectarian unrest could spread to volatile neighbouring countries.
Three rebel fighters were killed in clashes before dawn in Aleppo, the Observatory said.
It said 160 people were reported killed in Syria yesterday, adding to an overall death toll of around 18,000 since the uprising began.
Video footage provided by the Observatory showed smoke rising over apartment blocks in the city into a hazy sky. The sound of sporadic gunfire could be clearly heard.
Fighting was also reported in towns across Syria - in Deraa, the cradle of the revolution, Homs, the scene of some of the bloodiest combat, and Hama.
At least ten people were killed when Syrian security forces went into Maadameyat al-Sham near Damascus, the observatory said.
Meanwhile, Russian Foreign Minister Sergei Lavrov warned that international support for Syrian rebels would lead to "more blood".
He said that the government could not be expected to willingly give in to its opponents.
Mr Lavrov said Western and Arab nations should exert more influence on rebels to stop fighting.
The International Committee of the Red Cross has temporarily pulled some staff out of Syria because of escalating violence.
Elsewhere, French President Francois Hollande said he would try again to convince Russia and China to support harder sanctions against the Syrian president in order to break a diplomatic deadlock and prevent more bloodshed.
