Donald Trump has issued one of his most direct threats yet to Tehran, warning that “if Iran wants to fight, that will be the official end of Iran”.

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The US president emerged from his golf club in Sterling, Virginia, on Sunday to tweet belligerently at around 4.30pm, thereby risking a quickening of tension that is already rising.

“Never threaten the United States again!” he wrote.

The tweet will do little to assuage jitters in the Middle East and in Washington about aggressive language coming out of the White House. Concern is already running high that Trump’s hawkish national security adviser, John Bolton, who played a key role in instigating the invasion of Iraq under George Bush, might be nudging the administration towards military action.

In 2015, Bolton wrote a New York Times op-ed entitled “To stop Iran’s bomb, bomb Iran”. Trump withdrew the US from the Iran nuclear deal last year.

On the other hand, Trump has a way of blowing hot one minute and cold the next. As with so many of his social media missives, the precise import of his Sunday tweet was hard to read. It directly conflicted with reports of just three days ago that the president had been telling the Pentagon he did not want to go to war and wanted to find a way to wind down tensions.

Those reports were also subject to qualification. In response to reports about a draft plan for the deployment of 120,000 troops, Trump said that though he did not want war, if it came to it he would send “a hell of a lot” more soldiers than that.

Earlier on Sunday, the Utah senator and former Republican presidential candidate Mitt Romney had joined the commander of the Iranian Revolutionary Guards in dismissing the threat of war.

“Going to war with Iran?” Romney asked on CNN’s State of the Union. “Not going to happen.”

According to the Fars news agency, Maj General Hossein Salami followed foreign minister Mohammad Javad Zarif by saying Iran was not pursuing war either.

But both men offered caveats.

Romney, a member of the Senate foreign relations committee, said the threat to US interests was “real” and added: “We’re going to make sure they understand that if they take action against our people, against our allies and against our friends, there will be consequence and it will be far more severe than the initial action taken by Iran.”

Salami said Iran was ready to fight, as the difference “between us and them is that they are afraid of war and don’t have the will for it”.

The White House has not said what is behind its claim of an increased threat. Romney said the “intelligence community says there’s a great deal of risk” but did not elaborate. It has been reported that US intelligence believes Iranian commercial vessels have carried missiles and ammunition, which some analysts say indicates preparations to defend against a US attack.

Saudi Arabia is the major US ally in the region. Four oil tankers, two of them Saudi, were attacked off the coast of the United Arab Emirates. Iran-allied rebels in Yemen claimed a drone attack on a Saudi oil pipeline.

Adel al-Jubeir, Saudi Arabia’s foreign minister, told reporters on Sunday his country also “does not want war … but at the same time, if the other side chooses war, the kingdom will fight this with all force and determination”.

The US has sent an aircraft carrier strike group and cautionary moves include an evacuation of personnel by the oil firm ExxonMobil and a warning from the US to commercial air traffic of increased risk in the region.

The Associated Press reported on Sunday that Democrats in Congress will be briefed by former CIA director John Brennan, a stringent Trump critic, and Wendy Sherman, a former state department official who helped negotiate the Iran deal. Among Democratic presidential hopefuls on Sunday, the presumption was that Trump either wanted war or was behaving irresponsibly.

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The Hawaii congresswoman Tulsi Gabbard, a military veteran, told ABC’s This Week Trump was “leading us down this dangerous path towards a war in Iran”. Seth Moulton of Massachusetts, another veteran, said war with Iran would be “exactly what John Bolton wants”.

But David Petraeus, a retired general who led US troops in Iraq in 2003 and later led the CIA, told ABC it was “pretty clear” Trump “doesn’t want to go to war with Iran. He’s not after regime change”.

Romney agreed.

“I don’t believe for a minute,” he said, “that either the president or John Bolton or anyone else in a serious senior position of leadership in the White House has any interest in going to the Middle East and going to war. That’s just not going to happen … barring some kind of attack from Iran or something of that nature which I don’t think anyone anticipates.

“Look, the president made it very clear that he thinks the greatest foreign policy mistake probably in the modern age was the decision by President Bush to go into Iraq. The idea that he would follow that by going after Iran, a more difficult enemy if you will, that’s just not going to happen.”

Famously, Trump said at the time that he supported George W Bush’s invasion of Iraq. He has since vehemently denied that he did so.