WASHINGTON (AFP) - Recent Iranian air strikes against Islamic State jihadists in Iraq took place in an eastern region where United States warplanes do not operate, the Pentagon said on Wednesday, reflecting an accommodation between the two traditional rivals in their fight against a common enemy.

US defence officials said air raids by Iranian F-4 Phantom fighters over the weekend targeting were part of a pattern in which Iranian or American military advisers have carved out separate spheres in Iraq.

"It was in eastern Diyala province," Pentagon spokesman Steven Warren said of the air strike. "We have not had any air activity there." The bombing run in eastern Iraq marked the first time the Iranian air force had flown its F-4 fighters in a combat mission against the IS group, US officials said.

"This is the first time we've seen it," Colonel Warren told reporters.

A defense official said both Teheran and Washington want to avoid confrontations or accidents that could turn into an international crisis.

But the US-led coalition is ready to tolerate Iranian military advisers or aircraft in eastern or southern provinces with large Shiite populations.

"There's a tacit understanding we're not going to operate in the same space. And they're not targeting American forces," the official, who spoke on condition of anonymity, told AFP.

The British military, part of the US-led international coalition against the IS extremists, is also aware of Iranian forces in Iraq but does not cooperate with them, Lieutenant General Gordon Messenger told lawmakers in London.

"We've known that Iran has had forces deployed into Iraq for some time," Messenger said, adding that Britain does not "deconflict" with the country's forces - a term meaning to coordinate to avoid accidental encounters in the field.

"Certainly we as a nation do not deconflict with Iran in anything that we do," Messenger said.

"As for the presence of, and co-ordination, that was reported this morning, I can't comment on that." US officials hope the Iranian air raids will pile pressure on the IS militants, who seized large swathes of territory in Syria and Iraq earlier this year as Iraqi army troops retreated in panic.

After hundreds of US and coalition air strikes since August - and after weapons deliveries from both Tehran and Washington - the Iraqi army and Kurdish forces have made some modest advances but the IS group has yet to be driven out of from strongholds in the north and west.

US and allied aircraft renewed air raids in Iraq against IS over the past three days, the military's Central Command said in a statement.

American and coalition warplanes conducted 11 strikes in Iraq since Monday in the north and west, including four raids against IS extremists near Mosul, it said.