The Syrian state-run TV station, al-Ikhbariya, cited its correspondent in Deir Ezzor separately reported the attack, saying the building had been used as a prison for a "large number of civilians".
Mayadeen is a town in the Euphrates River Valley, 45 kilometres southeast of the provincial capital of Deir Ezzor.
In April, President Trump ordered a cruise missile strike in response to what Washington claimed was a gas attack carried out by Assad's forces that killed almost 90 people.
He said the new tactic might be a result of repeated US -led coalition airstrikes that have wiped out tens of IS fighters' gatherings. When asked by a reporter if he envisioned the entirety of the group's USA -supplied weapons would be taken from them following the end of the campaign against the Islamic State, Mattis said, "We'll do what we can".
It said the Mayadeen mission was "meticulously planned and executed to reduce the risk... to non-combatants".
The Britain-based Syrian Observatory for Human Rights says at least 15 IS jailers and fighters were also killed in the airstrike that happened on Monday in the Deir El-Zour province.
The building belonged to an al-Qaida-linked commander before it was seized by the IS group in 2014, according to the media outlet.
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French voters have traditionally rallied behind their new leader in the legislative elections that follow the presidential ballot. Pollsters' projected that as many as one-third of votes went to Mr Macron's camp in the first stage of the two-part election.
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Approval rating for President Trump at lowest since inauguration, CBS poll says
A gender divide remains: as they have from the start of his presidency, men are more likely than women to approve of the job Mr. Just 28 percent of respondents approved of the way he's handled the probe, according to CBS , while 63 percent disapprove.
Mattis said the USA has made it clear all along that, "we're going to equip them for the fight".
The allegations of casualties at the prison "will be provided to our civilian casualty team for assessment", he added.
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 army and its Russian ally.
The White House on Monday accused Assad's regime of preparing a potential chemical attack and warned it would pay a "heavy price", but Defence Secretary Jim Mattis said Washington was determined to keep a strict focus on fighting ISIS.
Mattis's remarks leave open the possibility that the SDF will likely continue to fight the Islamic State in northern Syria after the operation to retake Raqqa ends.
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.
"The White House must have solid intelligence about a possible Syrian sarin attack but why they chose to send [a] message to Assad and Putin via press release isn't clear", Daryl Kimball, the head of the Arms Control Association said in a tweet.