Turkey's Foreign Minister Mevlüt Çavuşoğlu said on Friday that Syrian regime airstrikes in eastern Ghouta were unacceptable, and called on Russia and Iran to put pressure on Bashar Assad.

"Russia and Iran need to stop the Syrian [regime]," Çavuşoğlu said, adding that an offensive by regime forces in the northern Syrian province of Idlib also violated an agreement between Turkey, Iran and Russia to stabilize the lines of conflict and open corridors for humanitarian aid.

The besieged enclave, just east of the capital, is supposed to be one of four "de-escalation zones" declared last year in a bid to reduce the bloodshed.

But Assad has intensified its bombing of the district's towns and is also conducting a major offensive in another zone, Idlib province in the northwest.