The strike was "probably" carried out by American drones, a source from among the local pro-government forces told Reuters, adding that the attack allegedly targeted Iraqi factions between Albu Kamal and Tanf, as well as Syrian military positions.
President Bashar al-Assad's army, with the help of Iran-backed militias including Hezbollah and Iraqi groups, drove Islamic State from Albu Kamal and its environs a year ago but the jihadists have since staged attacks there.
Syrian President Bashar Assad has blasted the USA presence in Syria, referring to it as an occupation.
"Thirty-eight non-Syrian fighters from regime loyalist militias were killed in the night-time raid on Al-Hari, on the Syrian-Iraqi border", said Observatory head Rami Abdel Rahman.
Israel is also suspected of carrying out numerous raids on Syrian government positions and Hezbollah weapons caches over the years, and last month announced unprecedented strikes on what it said were Tehran-operated bases in Syria.
The Iraqi PMF have demanded an explanation from the United States over the airstrikes, but USA defense officials have repeatedly stressed that neither US nor allied aircraft attacked PMF or Syrian regime forces.
However, U.S. Central Command said no member of the U.S. -led coalition carried out strikes in the area.
Twarit Vishwa: 5.9 magnitude quake hits Japan, damage buildings
The natural disaster struck with a magnitude of 5.9 at 7:58 a.m. local time, at a depth of 10 kilometers (6.2 miles). The Osaka prefectural government reported two deaths, and an Ibaraki city official confirmed a third.
ISIS fighters are close to losing their small pockets in east Syria after being besieged in the two provinces of Hasaka and Deir Ezzor, particularly in areas located east of the Euphrates River.
He said the United States was an occupying power in Syria and that his state supported "any act of resistance, whether against terrorists or against occupying forces, regardless of their nationality".
In Baghdad, Iraqi officials said Popular Mobilization Forces - the mostly Shiite state-sanctioned paramilitaries - came under attack south of the town of Qaim, just across the border from Boukamal.
The Syrian Observatory for Human Rights, a war monitor, said unidentified planes had struck Lebanon's Shi'ite Hezbollah and other allied foreign militias around Albu Kamal. "We're looking into who that could possibly be, but it wasn't the U.S. or the coalition".
Asked about the reported air strikes, an Israeli military spokeswoman said: "We do not comment on foreign reports".
Washington said the strikes were in retaliation for an attack on its own personnel and SDF forces. The overnight attacks took place on the western side.