Turkish President Recep Tayyip Erdogan accused Moscow of "not honouring" agreements made with Ankara for northwestern Syria where Russian and Syrian warplanes have increased their bombardment, in remarks published Wednesday.

"There have been agreements made with Russia. If Russia honours these agreements, we will do the same. But right now, unfortunately, Russia is not honouring these agreements," Erdogan said, quoted by the Hurriyet daily.

Despite being on opposite sides of the war, Damascus ally Moscow and rebel supporter Ankara have worked closely to resolve the conflict.

This was a rare critical remark from Erdogan who has often sought to keep good relations with Russian President Vladimir Putin since a 2016 rapprochement.

But violence has raged in the last rebel stronghold of Idlib in spite of a 2018 Turkey-Russia agreement signed to prevent a full-scale Syrian offensive.

The deal was made in the Russian resort of Sochi. There have been numerous ceasefires for the region home to some three million people, including one sponsored by Moscow and Ankara earlier this month.

Yet eight civilians were killed Thursday by Russian air strikes in Idlib, a region to which many people fled after being displaced from other former rebel-held areas. As part of the Sochi deal, Turkey set up 12 observation posts, one of which was surrounded by Syrian President Bashar al-Assad's forces in December.