Donald Trump’s nominee to become the first permanent US defence secretary in more than six months, has said that the US is not seeking war with Iran and needed to “get back on the diplomatic channel”.

Mark Esper, who is currently the army secretary, also told the Senate armed services committee that the Trump administration does not have congressional authority to wage war on Iran on the basis of the mandate issued by Congress in the immediate aftermath of the 9/11 attacks.

Other senior Trump officials have refused to rule out using the 2001 Authorization to Use Military Force (AUMF) for military action against Iran and the secretary of state, Mike Pompeo, has sought to link Iran to al-Qaida.

Esper, a military veteran and former arms industry lobbyist, will be the first Senate-confirmed defence secretary the Trump administration has had since James Mattis stepped down at the beginning of the year. His confirmation appeared almost certain on Tuesday at a mostly friendly Senate hearing where he received significant Democratic support.

Esper was unequivocal in saying that the AUMF referred to fighting non-state terrorist groups, and not entire nations like Iran. He added, however, that the president has constitutional authority to respond to an attack on US troops or interests.

But Esper stressed: “I agree we do not want war with Iran, we are not seeking war with Iran, we need to get back on the diplomatic channel.”

He said the US might be interested in negotiating a variant on the 2015 nuclear deal, the Joint Comprehensive Plan of Action (JCPOA), which included limits on Iran’s ballistic missile programme.

Trump took the US out of the JCPOA in May last year, and has been seeking to impose a campaign of “maximum pressure” on Tehran ever since. But on Tuesday, he insisted that he was not looking to oust the current regime.

“We’re not looking, by the way, for regime change,” he told journalists. “It doesn’t work out too well. We’re not looking for that at all. They can’t have nuclear weapons. They can’t be testing ballistic missiles, which right now under that agreement … they’d be able to do. We want them to get out of Yemen.”

Sitting at the cabinet table with Trump, Pompeo claimed that: “For the first time … the Iranians said they are ready to negotiate on their missile program.”

That was quickly denied by the Iranian mission to the United Nations, denouncing a press report suggesting that Tehran was open to missile negotiations, which may have been the source of Pompeo’s comments.

In a written statement, the mission said that the press report had misinterpreted comments by the Iranian foreign minister, Mohammad Javad Zarif, who told NBC News: “if the US wants to talk about missiles, it should stop selling weapons, including missiles, to regional states.”

The statement said the comments were not an actual offer to negotiate.

“Iran’s missiles and its missiles are absolutely and under no condition negotiable with anyone or any country, period,” it said.