Turkey looks for diplomatic solution to avoid 'massacre' in Syria's Idlib

The Trump administration declaration came after the Kremlin criticized Trump's warnings against the expected offensive in the northern province, where Syria's rebels are holed up and have come under intense airstrikes.

On Tuesday, the White House released an official statement condemning the attacks on Idlib and threatened military force if "chemical weapons" are used.

US President Donald Trump warned Assad on Monday not to "recklessly attack" Idlib and the White House strongly cautioned him on Tuesday against using chemical weapons.

The White House warned Syria's President Bashar Assad on Tuesday that if he chooses to use chemical weapons, the USA and its allies "will respond swiftly and appropriately", pushing back against an expected Syrian government offensive against an opposition stronghold. At least 13 civilians were killed in the Russian bombing, according to monitors.

Assad's government has been accused of repeatedly using chemical weapons during the seven-year-old war.

A Syrian man rides a motorcycle past a destroyed building in an area that was hit by a reported airstrike in the district of Jisr al-Shughur, in the Idlib province, September 4, 2018.

"Let's try to avoid that the last probably major battle of the Syrian territorial conflict...ends in a bloodbath", he told reporters in Geneva, insisting Russian Federation and Turkey held "the key for the soft solution to the Idlib issue".

Tuesday's bombardment hit several areas held by the jihadist-led Hayat Tahrir al-Sham alliance, among them the large town of Jisr al-Shughur, but also areas held by rival Turkish-backed rebels, including the town of Ariha.

The Syrian military has been sending reinforcements to the area for more than a month, and Russian Federation has ramped up its rhetoric.

On Wednesday, Putin's spokesman Dmitry Peskov said Idlib was the "subject of increased concern and worry" and the focus of a flurry of diplomacy.

De Mistura said there were about 10,000 fighters with UN-recognized terrorist organizations now in Idlib.

FILE - A Syrian girl holds an oxygen mask over the face of an infant at a make-shift hospital following a reported gas attack on the rebel-held besieged town of Douma in the eastern Ghouta region on the outskirts of the capital Damascus on January 22, 2018. "Let's go in. Let's kill the ... lot of them", Trump told Mattis on the phone.

Mattis told the president that he'd get right on it - but after hanging up, he told a senior aide: "We're not going to do any of that".