The involvement of external powers in the Syrian war continued to accelerate Tuesday as President Bashar Assad’s government and Russia dismissed White House allegations that it was preparing a new chemical weapons attack.

Hours later activists reported an airstrike on an Islamic State-run jail in eastern Syria that they said killed more than 40 prisoners and was carried out by the US-led coalition.

The Britain-based Syrian Observatory for Human Rights said at least 15 militants were also killed in the airstrike that happened on Monday in the Deir El-Zour province. The activist-run Deir Ezzor 24 media outlet said at least 60 civilians were killed.

Russia and Syria also carry out airstrikes in Deir el-Zour, and it was not clear how the activists identified the aircraft responsible. The coalition could not immediately be reached for comment.

Ali Haidar, the Syrian minister for national reconciliation, meanwhile dismissed a White House statement Monday that warned Syrian President Bashar Assad’s government against carrying out another chemical attack. Haidar told The Associated Press the charges foreshadowed a new diplomatic campaign against Syria at the UN.

The Kremlin also dismissed the White House statement, which had warned that Assad and his military would “pay a heavy price” if it goes ahead with the attack. Russian President Vladimir Putin’s spokesman Dmitry Peskov said that “such threats to Syria’s legitimate leaders are unacceptable.”

Russia is Assad’s key backer and sided with him when he denied responsibility for a chemical weapons attack that killed dozens of people in Idlib province on April 4. Days later, President Donald Trump ordered a retaliatory cruise missile strike on a Syrian air base.

Peskov criticised the Trump administration for using the phrase “another chemical weapons attack,” arguing that an independent investigation into the April attack was never conducted despite Russia’s calls for one.

The statement by White House Press Secretary Sean Spicer said the US had “identified potential preparations for another chemical weapons attack by the Assad regime that would likely result in the mass murder of civilians, including innocent children.”

He said the activities were similar to preparations taken before the attack in April, but provided no evidence or further explanation.

Several State Department officials typically involved in coordinating such announcements said they were caught completely off guard by the warning, which didn’t appear to have been discussed in advance with other national security agencies. Typically, the State Department, the Pentagon and US intelligence agencies would all be consulted before the White House issued a declaration sure to ricochet across foreign capitals.

The officials weren’t authorised to discuss national security planning publicly and requested anonymity.

A non-governmental source with close ties to the White House said the administration had received intelligence that the Syrians were mixing precursor chemicals for a possible sarin gas attack in either the east or south of the country, where government troops and allied forces have faced recent setbacks.

A senior Russian lawmaker dismissed the US warning as “provocation.”

Frants Klintsevich, first deputy chairman of the defense and security committee in the upper chamber of the Russian parliament, accused the United States of “preparing a new attack on the positions of Syrian forces.”

The US strike in April was the first direct American assault on the Syrian government and Trump’s most dramatic military order since becoming president.

Trump said at the time that the chemical attack crossed “many, many lines,” and called on “all civilized nations” to join the US in seeking an end to the carnage in Syria.

Syria denied using chemical weapons. Russia’s Defense Ministry said the toxic agents were released when a Syrian airstrike hit a rebel chemical weapons arsenal and munitions factory.

The US attack on a Syrian air base came after years of heated debate and deliberation in Washington over intervention in the bloody civil war. Chemical weapons have killed hundreds of people since the start of the conflict.

The US is providing air support and arms to Kurdish-led Syrian forces who are fighting to drive the Islamic State group from Raqqa, the extremists’ self-styled capital.

Defense Secretary Jim Mattis said Tuesday that Washington would continue to provide weapons after the Raqqa battle is over. His comments were likely to anger Turkey, which views the Kurdish fighters as an extension of the insurgency raging in its southeast.

On Monday, Trump had dinner with Mattis, Secretary of State Rex Tillerson, National Security Adviser HR McMaster and other top officials as he hosted Indian Prime Minister Narendra Modi at the White House.

Tillerson and Russian Foreign Minister Sergey Lavrov talked earlier Monday about the need to secure a cease-fire in Syria, fight extremist groups and prevent the use of chemical weapons, the Russian Foreign Ministry said.

Nikki Haley, the US Ambassador to the United Nations, followed up Spicer’s statement with a Twitter warning: “Any further attacks done to the people of Syria will be blamed on Assad, but also on Russia & Iran who support him killing his own people.”

Less than an hour after Spicer issued the statement, Trump was back to tweeting about the 2016 campaign, denouncing investigations into potential collusion between Moscow and his campaign aides as a “Witch Hunt!”