U.S. President Donald Trump warned Syria, Russia and Iran in a tweet on Monday against "making a grave humanitarian mistake" by attacking Syria's rebel-held 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 tweeted Monday evening.
Earlier Monday, Iran voiced support for an impending Syrian offensive to recapture the last remaining major opposition stronghold, saying the Idlib region in northwestern Syria should be cleared of "terrorists."
Iranian Foreign Minister Mohammad Javad Zarif traveled to Damascus on Monday to meet with his Syrian counterpart as well as President Bashar al-Assad.
Iranian media quoted Zarif saying Idlib should be put back under control of the Syrian people, and that reconstruction efforts in Syria and the return of refugees should go forward.
The country has been at war since early 2011 with a multitude of parties including pro-government forces, rebel groups and militants all fighting for control over various areas.
Assad's forces, backed by military support from Russia and Iran, have recaptured major cities in recent years, often involving agreements with both opposition fighters and civilians that allowed them to flee to Idlib.
Now the pro-government forces are focused on Idlib, where there are about three million and worries from the United Nations that an offensive there would bring a large humanitarian toll.
Iranian Foreign Ministry spokesman Bahram Qassemi told reporters in Tehran that Syria has every right to reclaim its territory from terrorists and Iran's support will continue.
Assad's government has long referred to any opposition fighters as "terrorists." The Idlib area includes both rebel groups and militants such as the Nusrah Front.
Zarif's visit comes ahead of a planned trilateral meeting in Tehran Friday between the presidents of Iran, Russia and Turkey to discuss the situation in Syria.