Washington sees a potential attack on Idlib as an escalation of an already dangerous conflict, US secretary says.

US Secretary of State Mike Pompeo has accused his Russian counterpart Sergey Lavrov of “defending the assault” by Syrian government forces on the last rebel stronghold in Idlib province.

Forces loyal to the Syrian President Bashar al-Assad have been massing around Idlib, which borders Turkey, for days and seem ready to launch what could be the last major battle of the civil war that has torn the country apart since 2011.

“Sergey Lavrov is defending Syrian and Russian assault on #Idlib,” Pompeo tweeted on Friday.

“The Russians and Assad agreed not to permit this. The US sees this as an escalation of an already dangerous conflict.

“The 3 million Syrians, who have already been forced out of their homes and are now in #Idlib, will suffer from this aggression. Not good. The world is watching.”

The 3 million Syrians, who have already been forced out of their homes and are now in #Idlib, will suffer from this aggression. Not good. The world is watching. — Secretary Pompeo (@SecPompeo) August 31, 2018

Turkey, Russia and Iran, which is backing Assad, all operate “observation points” in Idlib as part of a “de-escalation” deal agreed last year that was meant to reduce bloodshed in the province.

The prospect of a massive Russian-backed offensive in a province that is home to millions of people has raised fears of a new humanitarian tragedy.

Half of Idlib’s residents are displaced Syrians from other parts of the country.

During a press conference with his Saudi counterpart in Moscow on Wednesday, Lavrov hinted the assault may be imminent.

UN warnings

UN chief Antonio Guterres said on Wednesday he was “deeply concerned about the growing risks of a humanitarian catastrophe in the event of a full-scale military operation in Idlib”.

Filippo Grandi, the UN high commissioner for refugees, also warned on Friday that a potential government offensive in the province risks causing renewed displacement and discourage others Syrians from returning home.

Grandi, who made the comments in Lebanon after visiting Syria and Jordan, said an all-out attack threatens to cause many civilian deaths and fresh displacement, as well as discourage the return of other refugees.

“You risk also sending a message to refugees that the situation is not secured,” said Grandi.

“Refugees will be watching very closely what is happening in Idlib in the next few months.”

On Friday, Turkey, which backs other rebel groups in the province, officially designated Tahrir al-Sham, which has presence in Idlib, as a “terrorist” organisation.

Intense negotiations have been under way for weeks between Russia and Turkey, which hosts some three million Syrians and has already stated that it will not open its borders to accept further refugees in the event an assault takes place.