Turkey's Prime Minister Recep Tayyip Erdogan on Friday urged international action in Syria, saying it was not possible "to remain a spectator" to the regime's offensive on its second city Aleppo.
The Syrian regime has ramped up its pressure on the key northern hub over the last two days as world powers expressed fears of an all-out onslaught against rebel forces and civilians there.
Erdogan, speaking after meeting British Prime Minister David Cameron at his Downing Street residence, urged joint action from the UN Security Council, the Organisation of Islamic Cooperation and the Arab League.
"The most important (thing), which we have seen arise more recently, is the situation in Syria where what is happening is very important and very dangerous," Erdogan told a joint press conference with the British premier.
"There is a regime there that kills and massacres its own people.
"We must do what we can together, in the United Nations Security Council and also in the Organisation of Islamic Cooperation and the Arab League, to make sure that we can make some important progress in trying to avert this appalling situation.
"There is a build up in Aleppo and the recent statements, with respect to the use of weapons of mass destruction, are actions that we cannot remain an observer or spectator to."
Tanks, helicopter gunships and warplanes have poured into Aleppo over the last two days with troops firing on a string of rebel neighbourhoods in its southwest in the battle to control the commercial hub.
Cameron echoed his Turkish counterpart's concerns about the Syrian regime's military actions in the country's most populous city.
The British premier said he and Erdogan had discussed "the very real concerns that we have that the Syrian regime is about to carry out some truly appalling acts around and in the city of Aleppo.
"This would be completely unacceptable.
"This regime needs to realise it is illegitimate, it is wrong and it needs to stop what it is doing.
"The international pressure against this regime and against Assad is only going to build until he finally goes."
Aleppo has been the scene of fierce fighting since rebels launched a major offensive on July 20 from rear bases across the nearby border with Turkey.
UN Secretary General Ban Ki-moon earlier Friday urged President Bashar al-Assad's regime to stop its offensive on the city, while British Foreign Secretary William Hague warned the assault could spark a humanitarian disaster.