The international community has stepped up pressure on the Syrian government to end its assault on the country's biggest city, Aleppo, where a potential massacre is feared.
The pro-government al-Watan newspaper has warned that the "mother of all battles" is about to start. Heavy gunfire has been heard from Aleppo and a steady stream of vehicles carrying families is leaving the city.
Turkish prime minister Recep Tayyip Erdogan, speaking after meeting British prime minister David Cameron in London, urged the international community to make progress in its efforts to stop the violence.
"There is a regime there that kills and massacres its own people," he said.
"There is a build-up in Aleppo, and recent statements with respect to the use of weapons of mass destruction are actions that we cannot remain an observer or spectator to."
'Truly appalling acts' feared
Mr Cameron said he and Mr Erdogan had discussed concerns that the Syrian regime "is about to carry out some truly appalling acts around and in the city of Aleppo".
"This regime needs to realise it is illegitimate, it is wrong and it needs to stop what it is doing," Mr Cameron said.
"The international pressure against this regime and against [President] Assad is only going to build until he finally goes."
United Nations secretary-general Ban Ki-moon has urged the Syrian government to halt its offensive and also demanded a clear statement that it will not use chemical weapons under any circumstances.
Separately, the UN's high commissioner for human rights, Navi Pillay,has appealed to both sides to spare civilians, citing concerns of "the likelihood of an imminent major confrontation".
Conditions in city said to be dire
The BBC's correspondent on the Syria-Turkey border says conditions in Aleppo are reported to be dire. Thousands of government troops have been drafted in from other areas and are encircling the city, he says.
An activist in the city told the BBC at least 15 people had died on Friday morning local time during the military's bombardment of a building.
"The people of Aleppo are not coping with this crisis," said the activist. "They are dying. It is a massacre. People can leave their homes and move around the city but who would really want to take the risk of being shot or bombed?"
The Red Crescent has suspended some of its operations in Aleppo because of the heavy fighting.
Fall of regime seen as inevitable
Earlier, the former head of the UN observer mission in Syria said it was "only a matter of time" before the Assad government fell.
"It's impossible to imagine a future in Syria where the current people in power remain in power," Major-General Robert Mood told a news conference in Oslo.
But General Mood, who left Syria last week, said Mr Assad's fall would not necessarily mean an end to the 16-month-old conflict.