Helicopter gunships opened fire over Aleppo on Sunday and the thud of artillery boomed across neighbourhoods as government forces and rebels fought for control of Syria's second city.
Opposition activists reported clashes in several rebel-held districts in the morning in what could herald the start of a decisive phase in the battle for Aleppo, Syria's commercial hub.
International peace envoy Koffi Annan and other foreign leaders said the situation in Aleppo served to emphasise the need for a negotiated political solution to the conflict in Syria, now in its 16th month.
A Syrian opposition leader urged foreign allies to circumvent the divided U.N. Security Council and intervene directly in the struggle to topple President Bashar al-Assad.
"Our friends and allies will bear responsibility for what is happening in Aleppo if they do not move soon," said Abdelbasset Sida, the head of the Syrian National Council, the main alliance for opposition to Assad.
Assad's forces, including tank columns, have been massing for an assault to wrest back neighbourhoods from rebel units and skirmishing has been going on for days in districts such as southwestern Saleheddine.
One opposition activist said he had seen tanks and armoured troop carriers heading towards Salaheddine on Sunday.
The Syrian Observatory for Human Rights, a pro-opposition monitoring organisation, said fighting was taking place early on Sunday in central districts of the city and a neighbourhood on the western outskirts.
Rebel-held areas visited by a Reuters reporter appeared to be deserted by residents. Fighters were using houses - some of them clearly abandoned in a hurry, with food still in the fridges - as their bases.
A jet fighter flew over the city shortly after dawn.
On the approaches to Aleppo from the north, many villagers were still shopping or tending their fields. But fighters from the rebel Free Syrian Army were also in evidence.
One man in his 40s, carrying his family on a motorcycle, said he was fleeing the fighting in the city.
"We are living in a war zone," he told Reuters. "I and my relatives are just going back and forth, trying to stay away from the fighting. We left Aleppo when we saw smoke and helicopters firing."
The battle for the city of 2.5 million people is seen as a crucial test for a government that has committed military resources to holding control of its two main power centres, Aleppo in the north and the capital Damascus.
"If the regime is defeated, it will be a decisive stage for Syria," the Observatory's Rami Abdulrahman said.
While neither side has managed to gain the upper hand, the uprising is being watched anxiously outside Syria amid concern that sectarian conflict could spill over its borders.
Minority Alawites have dominated through more than 40 years of Assad family rule in Syria, which has a Sunni Muslim majority.
Military experts believe that while Assad's more powerful forces will overcome the rebels in Aleppo and other cities, it risks loss of control in the countryside because the loyalty of large sections of the army is in doubt.
The British-based Observatory said 26 people were killed in Aleppo on Saturday. Across Syria it reported fighting in Deraa, the cradle of the revolution, Homs, the scene of some of the bloodiest combat, and Hama, which killed a total of 190 people, including 41 soldiers.
Video footage provided by the Observatory showed smoke rising over apartment blocks in the city into a hazy sky on Saturday. The sound of sporadic gunfire could be clearly heard.
In Paris, French President Francois Hollande said he would keep trying to convince Assad's main defenders, Russia and China, which have Security Council vetoes, to support harder sanctions against him.
"I will once more address Russia and China so that they recognise there would be chaos and civil war if Bashar al-Assad isn't soon stopped," Hollande said.
The Syrian government knew it was doomed and would use force until the very end, Hollande said.
"The role of the member states of the U.N. Security Council is to step in as quickly as possible."
Russia played down speculation that it might offer Assad asylum, with Foreign Minister Sergei Lavrov saying on Saturday Moscow had no such agreement and was not even thinking about it.
Russia has said international support for Syrian rebels would lead to "more blood" and the government could not be expected to willingly give in to its opponents.
It has also said it would not allow searches of Russian-flagged ships under new European Union sanctions governing vessels suspected of carrying weapons to Syria.
The increase in fighting in Aleppo follows a bomb attack on July 18 that killed Assad's defence minister and three other senior officials in Damascus, a development that led some analysts to speculate that the government's grip was slipping.