WASHINGTON, Sept. 4 (Xinhua) -- U.S. Secretary of State Mike Pompeo on Tuesday spoke with Turkish Foreign Minister Mevlut Cavusoglu over the phone, and agreed that Syria's possible attack in its rebel-held Idlib Province is unacceptable.

According to a statement issued by U.S. State Department spokesperson Heather Nauert, Pompeo and Cavusoglu agreed that any military offensive by Syrian government troops in Idlib would be "an unacceptable, reckless escalation" of the conflict.

Pompeo "also stressed the importance of Turkey releasing Pastor Andrew Brunson," the statement read, referring to the U.S. pastor who was detained on spying charges.

Earlier on Tuesday, the White House said that it would "respond swiftly and appropriately" if Syria uses chemical weapons in the expected attack.

U.S. President Donald Trump tweeted Monday that "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!"

The Kremlin responded later that Trump's warning to Syria not to attack the rebel-held Syrian province ignored the danger terrorists pose there.

Kremlin spokesperson Dmitry Peskov said that Idlib will be one of the main issues on the agenda of the Russia-Turkey-Iran summit to be held in Tehran on Friday.

Since earlier this year, the Syrian rebels were driven out of key areas in the capital Damascus, the central province of Homs and the southern region.

The government's army has been amassing forces and military gears around Idlib in recent weeks, as the last major battle against the rebels in Syria looms.

Brookings Institution senior fellow Darrell West told Xinhua that "there is a high likelihood that the U.S. would launch counter-attacks against the Syrian military in order to stop the casualties."

"An American move would make an already tense region even more tense," West said. "It would run the risk of direct confrontation between the U.S. and Russian or Iranian forces, and that situation could escalate dramatically."

Idlib, with a population of about 2.5 million, is strategically important as it shares a border with Turkey to the north, and also hosts a Russian-operated air base. It is close to the coastal province of Latakia, home of Syrian President Bashar al-Assad.