"God forbid, a serious massacre could take place if there is a rain of missiles there," Erdogan told journalists on his plane after an official visit to the Central Asian state of Kyrgyzstan, Hurriyet daily reported.

His comments come as Syrian troops mass near the northwestern region of Idlib for a major assault that is raising fears of a humanitarian catastrophe on a scale not yet seen in the seven-year-old conflict.

Damascus and its main backer Moscow have vowed to root out the jihadist groups that dominate Idlib province.

Russia, an ally of President Bashar al-Assad's regime, on Tuesday resumed air strikes on Idlib after a 22-day pause.

Turkey, which backs some of the rebels, has held several rounds of talks with Russia aimed at averting an assault on Idlib, as well as with Washington.

"Our cooperation with Russia now is very important. The United States sends the ball into the corner of Russia and Russia into the corner of the US," he said.

Erdogan talked of a "very merciless process" unfolding in Idlib, warning of the risk that heavy fighting would trigger an influx of refugees.

"3.5 million people live there. God forbid, it is again Turkey where those people will flee in case of a disaster," Erdogan said.

Idlib is one of the so-called "de-escalation" zones set up as a result of talks by Russia, Turkey and Iran last year as Damascus regained control of more of the country.

Erdogan as well as the presidents of Iran and Russia will meet on Friday in Tehran for a summit expected to focus on Idlib.

"God willing we will succeed in averting the extreme actions of the regime there by getting a positive result from the Tehran summit," he said.

The UN Security Council is also due to meet Friday to address the situation in Idlib amid growing fears of an attack and the possible use of chemical weapons.

Washington warned Tuesday that if Damascus were to use chemical weapons, it would respond.