Iranian strikes on Islamic State targets in Iraq show that Tehran has become an unavoidable regional player but also highlight the unusual nature of the global fight against an extremist group that is uniting traditional foes, experts say.

Both Washington and Tehran have been at pains to stress there has been no military coordination between them in the battle against IS in Iraq.

But the Pentagon has revealed that Iranian F-4 Phantom jets hit IS fighters in eastern Iraq in recent days.

And US Secretary of State John Kerry hailed any action by Iran as "positive", showing the fight against Islamic State is bringing together strange bedfellows.

"The situation is extremely delicate," said Rosemary Hollis, professor of international politics at London's City University.

"For Iran and the United States to be on the same side against a Muslim organisation -- albeit a radical Salafist organisation that is using terror tactics against the population in Iraq -- this is too embarrassing to acknowledge in the open," she added.

Another expert, Denis Bauchard from the French Institute on International Relations, said that "paradoxically, Iran and the United States have basically become allies" against the Islamic State.

"This confirms what we already know: Iran is a key player in all the major regional issues," the expert told AFP.

And most experts believe that even if there is no direct military coordination between historic foes Iran and the United States, some degree of information sharing must be in place -- most likely via the Iraqi authorities.

Pentagon spokesman John Kirby already hinted as much.

"We are flying missions over Iraq. We coordinate with the Iraqi government as we conduct those. It's up to the Iraqi government to deconflict that air space," Kirby told reporters.

"Nothing has changed about our policy of not coordinating military activity with the Iranians," added Kirby.

Experts also point to the likelihood of close contact between authorities in Baghdad and Tehran and the fact that the US command centre based in Qatar would immediately detect any Iranian fighters.

- 'America's silent partner' -

Didier Billion, from the Paris-based Institute of International and Strategic Relations, said: "It's not yet an alliance, but it's certain that the Iranians have an interest in getting involved in international affairs."

"In effect, (Iran's supreme leader Ayatollah Ali) Khamenei has set Iran up as America's silent partner in the Middle East," wrote Michael Doran from the US Brookings Institute in a recent commentary.

Hollis noted that the Iranians were in fact quicker off the mark to help the Iraqis as Islamic State militants marauded through their land in a bid to create a self-declared "Caliphate".

Tehran "put some of their forces on the ground to help the Iraqis way back in the summer before the Americans got going with their bombing campaign," she said.

And despite sharing the common goal of destroying the Islamic State group, Washington and Tehran are still opposed on many other issues, notably Iran's disputed nuclear programme, experts stressed.

Both sides have to tread carefully. "Iran wants to act discretely so as not to appear dependent on a coalition run by the United States," said Bauchard. "It is a motley coalition, because its members have different goals."

Billion added it was a "complicated game."

"Let's not forget that in September, France wanted to invite Iran to an international conference on Iraq but ran up against a US and Saudi veto," he recalled.