International concern was mounting on Friday over a looming massacre as Syrian troops bombarded the besieged city of Aleppo with artillery, strafed it with aircraft and pulled in major reinforcements ready to crush the outgunned opposition fighters.
The battle is one of the most important of the 17-month-old uprising. With a population of about 3 million, Aleppo is Syria's largest city and commercial hub, a key pillar of support for Assad's regime.
"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," he said at a joint news conference in London with British Prime Minister David Cameron.
"Steps need to be taken jointly within the United Nations Security Council, the Organization of Islamic Cooperation, the Arab League, and we must work together to try to overcome the situation," said Erdoğan.
Western nations in the Security Council have been trying to pass a strong resolution condemning Syria and imposing harsher international sanctions. Russia and China have blocked them, citing concerns that they could pave the way for foreign military intervention - as was the case in Libya.
As a result, there is little the international community can do to help the rebels surrounded in Aleppo - or in other Syrian cities like Hama, Homs, Deir al-Zour and Daraa, where there are daily reports of bombardments and casualties. Activists estimate more than 150 people are killed every day nationwide. July is set to become the bloodiest month of an uprising that activists say has claimed 19,000 lives since it began on March 2011.
Gulf countries like Saudi Arabia and Qatar are believed to be funding the opposition fighters through Turkey and there have been indications that the ragtag Free Syrian army is using better weapons and tactics, as seen by their tenacious hold on neighborhoods in Aleppo and other cities.
The opposition fighters controlled several neighborhoods but were facing reports of troops and tanks massing outside the city. The nonstop fighting in Aleppo has already claimed the lives of at least 145 rebels and civilians in the last six days, according to the Britain-based Syrian Observatory for Human Rights.
Cameron said Britain and Turkey were concerned that Assad's government was about to carry out some "some truly appalling acts around and in the city of Aleppo."
"This would be completely unacceptable. This regime needs to realize it is illegitimate, it is wrong and it needs to stop what it is doing," Cameron said.