The Britain-based Syrian Observatory for Human Rights said the Wednesday strike was probably carried out by the coalition.

But the anti-jihadist alliance denied having carried out any raids in Syria or neighbouring Iraq on Wednesday night.

"The US/Coalition did not conduct any strikes in Syria or Iraq last night," a spokesman said in an statement to AFP.

Before the strike near the border town of Albu Kamal, rockets were fired at a military base north of Baghdad hosting coalition troops, killing two Americans and one Briton.

It was the deadliest such attack in years on an Iraqi military base hosting foreign troops.

There was no immediate claim of responsibility, but the United States has blamed Iran-backed factions from the Hashed al-Shaabi for similar violence in recent months.

Within hours, the air strikes were launched against Hashed forces just across the border in Syria.

Hardline Hashed factions have fought alongside Syrian government forces for several years and have been targeted by both coalition and Israeli air strikes.

Israel has carried out hundreds of strikes in Syria since the start of the war in 2011, which it says mainly target Iranian forces and allied Lebanese fighters.

The coalition has been carrying out air strikes in Syria since 2014, when the Islamic State group overran a large swathe of the country and neighbouring Iraq.

Coalition-backed forces last year expelled the jihadists from their last outpost in eastern Syria, but the group retains sleeper cells on both sides of the border.