US Defence Secretary Jim Mattis on Tuesday (September 11th) stressed that Syrian President Bashar al-Assad has been well warned against any chemical weapons use in Idlib province, AFP reported.

Regime forces on Wednesday shelled southern districts of Idlib province and adjacent opposition-held areas of Hama province, the Syrian Observatory for Human Rights said.

The Observatory did not immediately report any casualties from the bombardment.

The assault began hours after UN chief Antonio Guterres warned the Security Council any full-blown offensive in Idlib risks triggering a "bloodbath".

The Syrian regime has twice been targeted by US strikes after previous alleged chemical attacks, and US officials have in recent days said additional action would follow if al-Assad were to use the banned weapons in Idlib.

UN agencies and relief organisations have warned repeatedly that any major assault could spark one of the worst humanitarian disasters of Syria's war.

French Foreign Minister Jean-Yves Le Drian warned Tuesday that a regime offensive on Idlib could scatter thousands of foreign extremists abroad.

"In Idlib, we are watching very closely what the Assad regime, aided and abetted by the Iranians and the Russians are up to there," Mattis said.

Pointing to a US strike on a Syrian air base in April 2017 after al-Assad’s jets launched an alleged chemical attack in Idlib, Mattis noted that the Syrian leader suffered massive losses to his air force, losing around 17% of his fighter jets.

"He has been warned, and so we will see if he has wised up," he added.