U.S. Defense Secretary Ash Carter (L) listens as Chairman of the Joint Chiefs of Staff Army General Martin Dempsey testifies before a Senate Armed Services Committee in Washington March 3, 2015.

Iran’s role in an Iraqi military offensive to recapture Tikrit could be positive as long as it does not fuel sectarian divisions in the country, the U.S. military’s top officer said Tuesday.

General Martin Dempsey, chairman of the U.S. Joint Chiefs of Staff, told senators that Iran’s military assistance for Shiite militia was nothing new but was carried out in a more open manner this week as Iraqi forces pushed to retake Tikrit from Islamic State of Iraq and Syria (ISIS) jihadists.

“This is the most overt conduct of Iranian support,” Dempsey said, which came “in the form of artillery” and other aid.

“Frankly, it would only be a problem if it resulted in sectarianism,” he told the Senate Armed Services Committee.

U.S. commanders rarely discuss Iran’s activities in Iraq in public, stressing that Washington does not coordinate with Tehran’s military in any way — even though the two foes see the ISIS group as a common enemy.

U.S. officials have pressed the Shiite-led government in Baghdad to reach out to the country’s alienated Sunni community and worry that Shiite militia could persecute the Sunni community as they push to roll back the ISIS group.

In an assault launched Monday, officials in Baghdad say a 30,000-strong force has been mobilized to take back Tikrit.

Dempsey said Shiite militia — which are armed by Tehran — account for about two-thirds of the force while Iraqi government army troops make up the remainder.

If the Iraqi army and Shiite fighters “perform in a credible way” and defeat the jihadists in Tikrit, “then it will in the main have been a positive thing in terms of the counter-ISIL campaign,” Dempsey said, using an alternative acronym for the ISIS extremists.

At the same hearing, Defense Secretary Ashton Carter acknowledged Iraq did not ask for military support from Washington for the Tikrit operation, the largest assault so far by Baghdad against the IS.

Carter said he shared Dempsey’s concerns about sectarian divisions erupting and that Washington was closely monitoring the conduct of the campaign.

“I hope sectarianism does not show its ugly head,” Carter said.

The head of U.S. Central Command, General Lloyd Austin, said the American military was not surprised by the Iraqi army’s offensive on Tikrit or Iran’s role because American surveillance aircraft picked up preparations beforehand.

Austin, who oversees the U.S.-led air war against the IS in Iraq and Syria, said “we have good overhead imagery and those types of things.”

“So, you know, the activity in Tikrit was no surprise,” the general said. “I saw this coming many days leading up to this. It was a logical progression of what they’ve been doing in the east of the country.”

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