BOARD AIRCRAFT: The US military's top officer said Friday Iran was bolstering the firepower of Shia militias in Iraq but it remained unclear if Tehran was a help or a hindrance to the fight against Islamic State group jihadists.

General Martin Dempsey, speaking to reporters aboard his plane en route to a visit to Bahrain and Iraq, said he would raise his concerns about Iran's influence in talks with Iraq's leaders — days after Baghdad launched a large-scale operation to recapture Tikrit from the Islamic State group.

The Shia militias, armed and advised by Iran, are playing a major role in the Tikrit offensive.

But the US-led coalition — which has no dealings with the Shia fighters — has been markedly absent from the operation and allies fear Iran's activity could aggravate sectarian tensions.

Dempsey said US-led air strikes in recent months to the north near Baiji had put pressure on the IS extremists, laying the ground for the offensive on Tikrit.

“The Tikrit operation is only possible because of the air campaign we've been running around Baiji,” where the IS failed to retain control of oil refineries, Dempsey said.

“But I'm trying to get a sense for how our activities and their activities are complementary,” said Dempsey, referring to Iran's support for the Shia militias.

He said that “we're alert to the challenges of having Iran supporting Shia militia,” and Tehran's influence had sparked concerns in the anti-IS coalition, which includes Sunni Arab countries that view Iran as a threat.

It was not clear if Iran shared the same strategic goals as the Washington-led coalition, he said.

The international coalition was committed to a “unified” Iraq that represented the Sunni and Kurdish communities as well as the larger Shia population, he said.

“I want to make sure that those efforts can truly be complementary. If they can't, we're going to have a problem,” he said.

IS outnumbered in Tikrit

Tehran has provided the Shia militias with artillery, training, “some intelligence,” overhead surveillance “of the kind they're capable of providing”, he said.

But there was no sign that Iranian troops were directly “in the fight”, he said, adding he would ask about that in his meetings in Iraq.

“One of the things that the Iranian influence does is it probably makes the Shia militia more militarily capable. Undoubtedly,” the general said.

“But it also makes our coalition partners a little concerned.”

Dempsey said “the only one that can balance that is the prime minister of Iraq Haider al-Abadi”.

“I want to get his views on how he is seeking to balance that concern.”

He said it was only a matter of time before IS jihadists lost the battle for Tikrit.

“The numbers are overwhelming,” Dempsey said, adding that “hundreds” of IS fighters were facing an estimated 23,000 Iraqi army and Shia militia troops.

It was the first time that the Iraqi army and Shia militia “have been so intertwined in an operation”, he said.

Columns of Iraqi military trucks and armoured vehicles were lined up along the main road to Tikrit, and it resembled a rush hour traffic jam in Washington, “bumper to bumper”, he said.

But he said the real test would come after the town is recaptured, and how the Iraqi government treats the mostly Sunni population in the area.

“The important thing about this operation in Tikrit is less about how the military aspect of it goes, and more about what follows,” he said.

If Sunnis are allowed to return to their homes and “feel like the government is following an offensive with reconstruction and humanitarian assistance, then I think we're in a really good place”, Dempsey said.

But if Sunnis are mistreated or forced out, and if the Baghdad government fails to deliver humanitarian aid, “then I think we've got a challenge in the campaign”, he said.