U.S. President Donald Trump on Monday warned Syria against launching an attack on the country’s last 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 leader Bashar 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.

“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,” Trump tweeted.

“Hundreds of thousands of people could be killed. Don’t let that happen!”

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