Syrian government troops are reportedly massing around the city of Aleppo, with the US claiming they could be "lining up" for a massacre.
Forces loyal to Syrian President Bashar al Assad are said to be keeping up the attack on rebel-held positions in Syria's second city from the ground and in the air.
Fierce clashes in Aleppo yesterday killed more than 100 people, according to unconfirmed reports.
US State Department spokeswoman Victoria Nuland said: "Aleppo has again been bombarded by Syrian fighter jets in the latest desperate effort by the Assad regime to hold onto control and there are credible reports of columns of tanks to attack the city.
The State Department said that troop movements on Aleppo along with air strikes by helicopters and fixed-wing aircraft represented a "serious escalation" of the government's efforts to crush an armed rebellion.
"This is the concern: that we will see a massacre in Aleppo and that's what the regime appears to be lining up for," Ms Nuland added.
"Our hearts are with the people of Aleppo, and again this is another desperate attempt by a regime that is going down to maintain control, and we are greatly concerned about what they are capable of in Aleppo."
But Ms Nuland said the US did not foresee military intervention in the conflict without a mandate from the UN Security Council, where Russia has blocked US-led efforts to rally a stronger response.
Rebels also said a regime assault appeared imminent.
"The army's reinforcements have arrived in Aleppo," Colonel Abdel Jabbar al Okaidi, a spokesman for the rebel Free Syrian Army, was reported as saying.
"We expect a major offensive at any time, specifically on areas across the southern belt, from east to west."
UN Secretary General Ban Ki-moon and Foreign Secretary William Hague will meet in London today, with the crisis top of their agenda.
Mr Hague and Mr Ban are expected to discuss last week's failure to agree a Security Council resolution at the UN, after Russia and China vetoed moves to ramp up the pressure on Mr Assad.
Their meeting comes after Syria's most prominent defector, Republican Guard commander Brigadier Gen Manaf Tlass, who fled Syria earlier this month, put himself forward as someone to unite the fractured opposition groups trying to topple Mr Assad.
The factions met in Qatar yesterday to thrash out a deal over a transitional leadership.
Activists say 19,000 people have been killed since the uprising began last February as the Arab Spring pro-democracy uprisings swept the Middle East and North Africa.