He said through a tweet that the strikes could be a grave humanitarian mistake.

"President Bashar al-Assad of Syria must not recklessly attack Idlib Province. The Russians and Iranians would be making a grave humanitarian mistake to take part in this potential human tragedy. Hundreds of thousands of people could be killed. Don’t let that happen!" he said on Monday.

The US state department has vowed a strong response if Syria defies the threat and carries out the attack, which has been deemed as one that could have disastrous consequences for thousands of civilians by the United Nations.

US ambassador to the UN Nikki Haley tweeted: "All eyes on the actions of Assad, Russia, and Iran in Idlib. #NoChemicalWeapons"

Reports said that presently there are about 30,000 al-Nusra and al-Qaeda militants in Idlib. However, the province is also home to thousands of civilians.

Quoting sources, Reuters news agency said that Syrian president Assad was preparing a phased offensive.

He has the backing of both Russia and Iran, Syria's closest allies.

Answering in the affirmative, Russia has said that the militants must be wiped out from the province and has said that drills, such as those proposed by Syria, are just.

The UN has lauded the Syrian approach towards tacking terrorism, but has expressed doubts on how it would be done without hurting the civilians.

UN envoy to Syria Staffan de Mistura has volunteered to go to Syria and help in the evacuation process. Image: UN Photo/Violaine Martin

"There is a perfect storm based on warnings, counter-warnings which is gathering around and due to the dilemma, which is a true dilemma on how to defeat terrorists in Idlib and at the same time avoid affecting a huge number of civilians," UN envoy to Syria Staffan de Mistura said.

"So, while we are aware that efforts and discussions are taking place to avoid the worst-case scenario, one cannot ignore that miscalculations may indeed occur leading to unforeseen escalation and we are all very much concerned."

He has urged that a humanitarian corridor be set up for the evacuation of civilians.

Meanwhile, experts have warned that any strike carried out in Idlib will sour Syria's relation with Turkey, which maintains observation posts around the province.

According to Fars news agency, Iran is scheduled to hold a meeting on Friday, where leaders from the host nation, Russia and Turkey will discuss Idlib.

Image: Donald Trump/Twitter