Idlib tragedy: Trump warns Syria, Russia and Iran.

US President Donald Trump tweeted on Monday that "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!"

Trump warned Syria on Monday  against launching an attack on the country's last opposition and rebel stronghold with the help of Russia and Iran, saying the offensive could trigger a "human tragedy."

The warning came as Iran's foreign minister Mohammad Javed Zarif met with Syrian regime leader Bashar al Assad in a surprise visit to Damascus ahead of the looming offensive.

Syrian forces are amassing around the northwestern province of Idlib, in preparation for the assault.

The United Nations and aid groups have warned that a full assault on Idlib could spark a humanitarian catastrophe on a scale not yet seen in Syria's seven-year-old conflict.

But Russia and Iran have insisted that extremist groups in Idlib must be defeated and are expected to back regime forces in any assault.

Zarif's trip to war-ravaged Syria also comes just days before a top-level tripartite meeting in Tehran to discuss the Syrian conflict, now in its eighth year.

He met Assad to discuss "issues on the agenda for the tripartite meeting," according to the regime presidency's account on the Telegram messaging app.