Syrian state media say US-led coalition aircraft bombed pro-government forces near the border with Iraq on Sunday, leaving a number of people dead.
"At 22:00 last night a US plane hit a fixed headquarters of the Popular Mobilisation Forces' 45th and 46th brigades defending the border strip with Syria using two guided missiles which led to the martyrdom of 22 fighters", it said in a statement.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said it had documented the deaths of 38 non-Syrian militia fighters, but that it did not know whether they were Iraqi or Lebanese nationals.
Syrian and Iraqi forces have driven ISIS from virtually all the territory it once held in both countries, but the terrorists still control some remote areas along the border.
Col. Sean Ryan, a spokesman for the coalition, said it was aware of the reports but had not carried out any strikes in the area.
Throughout Syria's seven-year war, Israel has carried out scores of strikes within the neighboring country against what it describes as Hezbollah or Iranian targets.
The U.S.is leading a coalition backing Syrian Arab and Kurdish militias fighting against the Islamic State northeast of Albu Kamal.
The two forces have mostly avoided crashing into each other thanks to a de-confliction line that runs across the province along the winding Euphrates River.
Syrian troops are battling IS on the western river bank, while the US-backed Syrian Democratic Forces fight on the east.
Iraqi warplanes also have occasionally bombed IS positions in Syria's east.
Last month a dozen pro-regime fighters were killed in an air strike on Syrian government positions that the Observatory and state media blamed on the coalition.
In February, dozens of pro-government fighters - many of them Russian mercenaries - were killed in air and artillery strikes carried out by USA forces in response to what they said was an attack on their allies' headquarters.
Al-Hari lies on the western side, close to the river and the de-confliction line.
Syria's conflict began in 2011 with protests against Assad, but then spiralled into a full-blown war that has drawn in world powers and given rise of jihadists like IS.
The village had been one of the last IS-controlled areas in a corridor linking Syria with Iraq.
President Donald Trump in March said that the U.S.