ISTANBUL — Turkish President Recep Tayyip Erdogan warned the last rebel stronghold of Idlib in northwest Syria was on the verge of destruction as a barrage of airstrikes have killed hundreds of civilians since April and another tentative cease-fire is tested by lethal clashes.

Erdogan is the most outspoken supporter of the remnants of an opposition battling President Bashar al-Assad’s forces in the eight-year war, yet has also worked closely with Assad’s main backers, Russia and Iran, in a series of efforts to forestall an all-out attack on Idlib, a province adjoining Turkey where some three million people live.

But numerous cease-fires for Idlib have all collapsed and the Syrian army, backed by Russian airpower, has made deep advances into the region. The Syrian Observatory for Human Rights said on Wednesday at least eight soldiers and opposition fighters were killed on Tuesday, just three days after Russia put into force the latest truce. Russian reconnaissance flights resumed in the skies above on Wednesday, the UK-based monitor also said.

“Idlib is slowly being annihilated. Just as Aleppo was annihilated and leveled, now Idlib is in the same situation. It is not possible for us to remain silent in the face of all of this,” Erdogan said late on Tuesday at a press conference in Ankara. He said the airstrikes have killed 700 civilians in Idlib.