Turkish Prime Minister Recep Tayyip Erdogan said on Friday that international steps must be taken to deal with President Bashar al-Assad’s military build-up around the Syrian city of Aleppo and his government’s threat to use chemical weapons.

“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,” Reuters quoted Erdogan as having 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 Countries, the Arab League, and we must work together to try to overcome the situation,” he added.

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.

Earlier Friday, United Nations Secretary-General Ban Ki-moon said he was “deeply” concerned about reports of the possible use of chemical weapons by Syria, and demanded the government state it would not use them “under any circumstances.”

Syria admitted this week it has chemical weapons and said it will use them if attacked. The regime promised it would not engage in chemical warfare against rebels, saying it would only use chemical weapons if “Syria is exposed to external aggression.”

The United States said on Thursday it appeared that forces loyal to Assad were “lining up” for a massacre in the city of Aleppo, but again ruled out military intervention in the conflict.

The U.S. State Department said that credible reports of tank columns moving on Aleppo along with air strikes by helicopters and fixed-wing aircraft represented a serious escalation of the government’s efforts to crush an armed rebellion.