The White House on Tuesday issued a clear warning to Syrian President Bashar al-Assad that he must not use chemical weapons during the massive assault on Idlib province he is expected to launch in the coming days.

The White House statement warned Assad and his allies against the “reckless escalation of an already tragic conflict” that would risk hundreds of thousands of civilian lives.

“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 White House said.

Observers in Idlib, the last province of Syria held by rebel forces, report an increasing tempo of artillery barrages from Syrian troops and airstrikes blamed on Russia. The most recent report said eight civilians were killed by Russian airstrikes, five of them children.

Russian officials responded to President Donald Trump’s warnings about a humanitarian disaster in Idlib by saying the United States and its allies underestimate the danger of allowing terrorist forces to keep strongholds in Idlib.

Foreign Minister Sergey Lavrov went further in a Russian television interview on Tuesday and accused the United States of protecting the Jabhat al-Nusra terrorist organization, which is al-Qaeda’s franchise in Syria.

Syrian news agency SANA summarized Lavrov’s remarks on Wednesday:

Lavrov called upon the administration of US President Donald Trump to not repeat the mistakes of his predecessor Barack Obama whose team officially pledged to separate Jabhat al-Nusra terrorist organization from the other armed organizations, but it didn’t do anything to achieve that. He said that the three guarantor states of Astana process, namely (Russia, Iran and Turkey) want to end the war in Syria and to create the suitable conditions to enable the Syrians to choose their fate by themselves without any foreign interference or any attempt to impose solutions on them. He underlined the importance of the participation of the Russian forces in the military operation against terrorist organizations in Syria. He added that no one of those who speak about the issue of saving lives of people and respecting the sovereignty and territorial integrity of states has done anything to prevent the attempts of terrorists to establish control over Damascus.

SANA also claimed on Wednesday that “terrorists” left behind a cache of weapons and supplies as they retreated from villages in the Quneitra region that included U.S.- and Israeli-made weapons.

“Amounts of medical supplies and medicines were also discovered in the area in addition to equipment for the so-called ‘White Helmets,’ including fake blood, cosmetics and plastic body organs used for media fabrications,” the SANA report added.

The White Helmets are a volunteer civil defense and rescue operation in Syria lauded in the West for risking lives to rescue civilians after brutal Syrian government airstrikes and chemical weapons attacks.

The group was nominated for a Nobel Peace Prize in 2016. Syrian and Russian propaganda portrays the White Helmets as a subversive organization sponsored by foreign governments to fake civilian casualties and slander the Assad regime.

Last week, the Russian Ministry of Defense distributed propaganda accusing rebel fighters and British special forces operatives of planning a chemical weapons attack that the White Helmets would film and blame on the Syrian military to create a pretext for the United States and its allies to attack Assad’s forces. This disinformation campaign was taken as an ominous sign that Russia and Syria are planning a bloodbath in Idlib.

On Wednesday, U.S. Defense Secretary Jim Mattis said there is “zero intelligence that shows the opposition has any chemical capability” in Idlib province.

“We have made very clear that by putting out innuendo that somehow any chemical weapon use coming up in the future could be ascribed to the opposition – well, we want to see the data. We cannot see anything that indicates the opposition has that capability,” he said.

Mattis added that the United States is “very alert” to the danger of Syrian government forces deploying chemical weapons during the Idlib assault.

Turkish President Recep Tayyip Erdogan, who has been at odds with the U.S. government in several other areas, joined the Trump administration in warning Syria, Russia, and Iran to tread carefully in Idlib.

“Turkey is working with Russia and Iran. Turkey has 12 observation posts in Idlib, and our security forces are resolutely carrying out their duties. There are 3.5 million people [in Idlib]. God forbid, if this area is hailed by missiles there would be a grave massacre,” Erdogan said on Wednesday.

“In that case, where would those fleeing go to? The majority would come to Turkey. This could give rise to a new issue,” he warned, making his government’s interest in avoiding an Idlib disaster clear. He also expressed displeasure with progress on a U.S.-Turkish agreement to move Kurdish militia forces away from Syria’s border with Turkey.

Erdogan urged Iran, Russia, and Syria to proceed with a summit scheduled for September 7 and continue working on a diplomatic resolution to the Syrian conflict.

“I hope we will be able to prevent the Syrian government’s extremism in this region,” he added.