America's top military officer has ratcheted up the pressure on Iran by issuing an unusual public warning that the Pentagon is planning for “potential military courses of action”.

Admiral Mike Mullen, chairman of the joint chiefs of staff, blamed the Iranian government and Quds force of the Iranian Revolutionary Guard for its “increasingly lethal and malign influence” in Iraq. He said conflict with Iran would be “extremely stressing” for America’s overstretched forces, but added: “It would be a mistake to think that we are out of combat capability.”

Mullen said he was increasingly concerned about Iran’s growing involvement in supplying munitions and training to rebel Shi’ite militias and “killing American and coalition soldiers in Iraq”.

Speaking at a Pentagon news conference late on Friday, he said recent operations in the southern port city of Basra had revealed “just how much and how far Iran is reaching into Iraq to foment instability”. A Pentagon source said the admiral’s frankness was “extremely significant” and could pave the way for some form of attack on Iran. However, Mullen said: “The solution right now still lies in using other levers of national power, including diplomatic, financial and international pressure.”

Mullen’s tough rhetoric came shortly after General David Petraeus, the US commander in Iraq responsible for the troop surge, briefed Congress about the “nefarious activities” of the Quds force in stirring violence in Iraq. There were a total of 923 civilian deaths in Iraq last month, the highest number since August 2007.

“We should all watch Iranian actions closely in the weeks and months ahead, as they will show the kind of relationship that Iran wishes to have with its neighbour,” Petraeus said.

Petraeus was nominated last week to take over as commander of all US forces in the Middle East from Admiral William Fallon, who resigned in March after becoming an outspoken critic of American policy towards Iran.

Petraeus has been asked to prepare a briefing on the extent of Iranian involvement in Iraq. It will include the recovery of weapons with date stamps showing that they were recently manufactured in Iran.

American officers claim that Iran is responsible for new, highly dangerous roadside bombs in Iraq and accuse Iranian-trained militants of responsibility for the deadliest rocket and mortar attacks on Baghdad’s green zone.

“The question is not if Iran is unhelpful in Iraq,” said Philip Crowley, a retired air force colonel and defence expert at the Center for American Progress in Washington. “The question is what to do about it.” Robert Gates, the US defence secretary, said last Monday that Iran is “hell-bent on acquiring nuclear weapons”. He added that war would be “disastrous” but the military option must remain on the table. However, a senior defence source said the administration regarded Iran’s nuclear programme and its interference in Iraq as separate problems, requiring different tactics and solutions.

Moqtada al-Sadr, the Iranian-backed radical Shi’ite cleric, called on his followers at Friday prayers to stop fighting Iraqi troops and unite against “the occupiers” – US troops.

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