The Kremlin on Tuesday rejected President Donald Trump's warning that an attack on the last major enclave of Syrian rebels would be a "grave humanitarian mistake," saying the Idlib rebels are hindering efforts at a diplomatic solution to the crisis.

"President Bashar al-Assad of Syria must not recklessly attack Idlib Province," Trump tweeted. "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!"

Within hours of Trump's tweet, Russian warplanes reportedly carried out airstrikes against the rebel positions. Reuters, citing a report from the Syrian Observatory as well as at least one rebel leader, said airstrikes began Tuesday near the city of Jisr al-Shughour in the province that borders Turkey.

Assad has been building up forces for a phased offensive aimed at taking control of the province in northwest Syria. Kremlin spokesman Dmitry Peskov said the rebel drone attacks from Idlib pose a threat to Russia's temporary bases in the war-battered country.

"It goes without saying that this problem must be straightened out," Peskov said. "We do know that the Syrian armed forces are getting ready for tackling this problem."

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Peskov called Idlib home to a "nest of terrorists," but it is also home to more than 2 million refugees. The White House issued a statement saying it was monitoring the activity in Idlib.

"Let us be clear, it remains our firm stance that if President Bashar al-Assad chooses to again use chemical weapons, the United States and its allies will respond swiftly and appropriately."

The rise in tensions comes ahead of meetings aimed at creating a commission to help draft a new Syrian constitution. Representatives from Turkey, Russia and Iran will meet with Staffan de Mistura, the UN special envoy on Syria, early next week. A few days later Mistura will meet with envoys from the U.S., U.K., France, Saudi Arabia, Jordan, Germany and Egypt.

Mistura, in Geneva, echoed Trump's call for calm, saying violence could "hold hostage" efforts at a political settlement. The International Rescue Committee also called on the Syria, Russia and its allies to halt the attacks.

“The world is now watching with bated breath to see what unfolds," said Lorraine Bramwell, the IRC's Syria director. "Many civilians in Idlib have survived intense bombardment or fighting elsewhere in Syria and are rightly terrified about what they may now face."

The war has dragged on for seven years, cost hundreds of thousands of lives and driven millions from their homes. Syrian, Russian and Iranian forces have consistently battered Islamic State forces in the region, but also have battered the West-backed rebels who had hoped to drive Assad from power.

The Trump administration has largely stayed out of the fray, with coalition forces conducting bombing raids against Islamic State positions but avoiding clashes with Assad's armed forces or their Iranian and Russian backers.