Turkish PM, Recep Tayyip Erdogan
The international community has stepped up pressure on the Syrian government to end its assault on the country's biggest city, Aleppo.
The aerial bombardment of Aleppo intensified on Saturday morning, activists said, while clashes were reported in central districts, reports the BBC.
Western nations have warned of a potential massacre.
Turkish PM Recep Tayyip Erdogan said it was not possible "to remain a spectator" to events in Aleppo.
UN Secretary General Ban Ki-moon urged the Syrian government to halt its offensive. He also demanded a clear statement that it would not use chemical weapons under any circumstances.
"The violence from both sides must stop for the sake of the suffering civilians of Syria," he said in London.
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."
Syria has implicitly acknowledged that it has chemical weapons but says it will not use them against its own people, only against foreign invaders.
Cameron said he and 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," he said.
"The international pressure against this regime and against [President] Assad is only going to build until he finally goes."
Separately, UN High Commissioner for Human Rights Navi Pillay appealed to both sides to spare civilians, citing concerns of "the likelihood of an imminent major confrontation".
The BBC's Wyre Davies, 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.
The pro-government al-Watan newspaper warned that the "mother of all battles" was about to start.