"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. There is a regime there that kills and massacres its own people," Erdogan said after meeting Prime Minister David Cameron in London.
"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," the Turkish PM said.
Erdogan urged the UN Security Council and the Arab League to take necessary steps to bring the situation under control.
Cameron also insisted that the violence in Aleppo was unacceptable.
"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," said Cameron.
UN High Commissioner for Human Rights Navi Pillay also expressed concern that a major confrontation was imminent in Aleppo. Pillay appealed to both sides to halt the bloodshed.
The US had expressed fears that the regime was amassing forces to prepare for a massacre in the city. Helicopter gunships and tanks along with scores of troops flooded the commercial capital of Syria on 26 and 27 July.
Shelling in the region has not stopped even during nights. More troops are being brought in from other places.
Several disturbing videos have been uploaded by the activists depicting the horror of the violence. The videos are unverified.
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