The UK-based Syrian Observatory for Human Rights said the air strike took place on Monday at dawn, hitting a building in the town of Mayadin, south of Raqqa, that was being used as a prison.
Russian President Vladimir Putin's spokesman Dmitry Peskov says that "such threats to Syria's legitimate leaders are unacceptable".
It may also have been aimed at the regime's backers in Moscow and Tehran, who have resolutely backed Assad and denied the regime's responsibility for chemical weapons use.
Although the focus of U.S. military operations in the region is the defeat of Isis in its two major strongholds, Mosul in Iraq and Raqqa in Syria, the Trump administration has showed itself willing to act if the Assad regime carries out a major chemical weapons attack.
It said the allegation will be provided to its civilian casualty team for assessment and the results released in a monthly civilian casualty report release with credible and non-credible allegations.
Separately, Syrian state-run TV station al-Ikhbariya cited its Deir al-Zor correspondent as saying coalition warplanes had destroyed a building in al-Mayadeen used as a prison by Islamic State to hold a "large number of civilians". Still, on June 18, a U.S. Navy F/A-18E Super Hornet shot down a Syrian Sukhoi Su-22 bomber that attacked U.S. -backed rebel forces.
Unconfirmed reports from both activist groups have alleged that a US -led coalition was responsible for the strike.
Among the dead are at least 60 civilians, the activist-run Deir Ezzor 24 media outlet reported.
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Democrats won 37 of 65 House seats, theoretically five more than would be expected based on their statewide vote share. Colorado's map was drawn by a Democratic-dominated commission that Republicans criticized as "politically vindictive".
The coalition could not immediately be reached for comment.
Most of Deir Ezzor province is controlled by the militants and it has been the target of air strikes by both the coalition and the Syrian regime army and its Russian ally. Ali Haidar, the Syrian minister for national reconciliation, told The Associated Press that the charges foreshadowed a new diplomatic offensive at the United Nations against the Syrian government.
The White House statement on Monday night was made without forewarning and caught State Department officials by surprise.
The White House on Monday accused Assad's regime of preparing a potential chemical attack and said it would pay a "heavy price", prompting criticism from regime allies Russian Federation and Iran.
It said at least 42 people were killed, identifying them as prisoners of Islamic State.
Spicer added the activities were similar to preparations taken before an April 2017 attack that killed dozens of men, women and children.
Mattis did not address the nature of the intelligence or White House warning as he flew to Europe for a North Atlantic Treaty Organisation meeting, but said the United States was not going to get pulled into the conflict between the regime and the armed opposition.