US President Donald Trump has claimed that attacking militant-held Idlib would be a “humanitarian mistake” for Tehran and Moscow.

“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!” claimed the US president in a tweet on Monday.

His tweet followed the latest visit to Damascus by Iranian Foreign Minister Mohammad Javad Zarif to meet Syrian President Bashar al-Assad and other top officials from the war-ravaged country.

Trump’s comment follows similar remarks by US Secretary of State Mike Pompeo and US ambassador to the United Nations Nikki Haley.

Tehran is set to host an upcoming summit with Russian and Turkish leaders over the situation in Syria on September 7.

Upon arrival in the Syrian capital earlier on Monday, Zarif asserted that “Terrorists operating in the remaining parts of Idlib must be cleansed, and the region be returned to the people of Syria,”

Last week, Iranian Defense Minister Amir Hatami visited Damascus, where he signed a “defense and technical agreement.”

Read More:

• US prepared initial list of Syria targets for possible strike: Report
• Russia says Syrian army has every right to chase militants out of Idlib
• Turkey says working with Iran, Russia to avoid Idlib 'disaster'

The Syrian army, meanwhile, is gearing up for a major offensive to retake the last terrorist stronghold, but it is facing threats of being hit by the US and its allies.

yria has been gripped by foreign-backed militancy since March 2011. The Syrian government says the Israeli regime and its Western and regional allies are aiding Takfiri terrorist groups wreaking havoc in the country.