Reuters reports: President Bashar al-Assad’s artillery continued to pound rebel-held areas in and around Aleppo in preparation for an onslaught on Syria’s biggest city, while neigbouring Turkey called for international steps to deal with the military build-up.

Opposition sources said the shelling was an attempt to drive fighters inside Aleppo from their strongholds and to stop their comrades outside the city from resupplying them.

“They are shelling at random to instil a state of terror,” said Anwar Abu Ahed, a rebel commander outside the city.

The battle for Aleppo, a major power centre that is home to 2.5 million people, is being seen as a potential turning point in the 16-month uprising against Assad that could give one side an edge in a conflict where both the rebels and the government have struggled to gain the upper hand.

Turkish Prime Minister Recep Erdogan said late on Friday that international institutions needed to work together to address the military assault on Aleppo and Assad’s threat to use chemical weapons against external threats.

“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 Organisation of Islamic Countries, the Arab League, and we must work together to try to overcome the situation,” he said.

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”.

Turkey, a former ally of Assad and now one of his fiercest critics, cheered on the rebels in Aleppo.

“In Aleppo itself the regime is preparing for an attack with its tanks and helicopters … My hope is that they’ll get the necessary answer from the real sons of Syria,” Erdogan said earlier in remarks broadcast on Turkish TV channels.