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Beirut • The United States said Tuesday that it had observed activity involving chemical weapons at a Syrian air base attacked by U.S. forces earlier this year, suggesting that President Bashar Assad's government was preparing fresh strikes on the rebel-held north of the country.

The White House warned late Monday that the Assad government would pay a "heavy price" for such a strike, indicating publicly for the first time that the Syrian army still possessed the capacity to launch new chemical attacks against opposition areas.

Marine Maj. Adrian Rankine-Galloway, a Pentagon spokesman, said Tuesday that the undisclosed activity was centered at least in part on one aircraft hangar at the central Shayrat air base that Tomahawk missiles hit in a barrage of strikes on April 7 that marked the first U.S. military intervention against Assad's forces during six years of war.

Those strikes came after Assad's military dropped sarin on the northern town of Khan Sheikhoun, killing scores of civilians and leaving hospitals overflowing with hundreds more casualties.

U.S. military officials declined to say what kind of chemical weapons may be at Shayrat now, or how they observed them. The United States regularly flies both manned and unmanned aircraft over Syria and also has some satellites capable of recording images of the battlefield.

In Moscow, Kremlin spokesman Dmitry Peskov said in a conference call with reporters that Russia has no information about the impending threat of a chemical weapons attack and warned that any retaliation against the Assad government would be "unacceptable."

The Russian military intervened to shore up Assad's crumbling armed forces in 2015, and Moscow has never accepted the coalition's conclusion that the Syrian government was responsible for the chemical weapons attack in Khan Sheikhoun.

Peskov said Tuesday that "it is impossible, unlawful and absolutely wrong from the point of view of achieving a final Syrian settlement to put the blame on al-Assad without holding an inquiry."

Separately on Tuesday, the U.S.-led coalition said it was investigating reports that airstrikes targeting Islamic State infrastructure in the eastern city of Mayadin had also killed scores of civilians.

In a statement, the coalition said the mission on Sunday and Monday had been "meticulously planned and executed to reduce the risk of collateral damage and potential harm to non-combatants."

But monitoring groups said the strikes had also destroyed an underground prison holding at least 57 people who had been arrested for flouting the laws of Islamic State's hard-line rule.

The Islamic State is believed to have moved most of its leadership to Mayadin in Syria's Euphrates Valley, southeast of the group's besieged capital Raqqa, according to U.S. intelligence officials.

Omar Abu Layla, the director of the Deir Ezzor 24 news network, said that many of the civilian dead had been buried in mass graves.

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Lamothe reported from Washington. David Filipov in Moscow also contributed to this report.