The United States ordered the departure of non-emergency U.S. employees from its diplomatic missions in Iraq on Wednesday in another show of concern about alleged threats from Iran.

U.S. President Donald Trump's administration is applying new sanctions pressure on Tehran and sending additional forces to the Middle East to counter what it says is a heightened threat from Iran to U.S. soldiers and interests in the region.

Iran called it "psychological warfare," and a British commander cast doubt on U.S. military concerns about threats to its roughly 5,000 soldiers in Iraq, who have been helping local security forces fight ISIS jihadists.

The U.S. State Department said employees at both the U.S. Embassy in Baghdad and its consulate in Erbil, capital of semi-autonomous Kurdistan region, were being pulled out immediately due to safety concerns.

It was unclear how many personnel were affected, and there was no word on any specific threat. Visa services were suspended at the heavily fortified U.S. missions.

"Ensuring the safety of U.S. government personnel and citizens is our highest priority ... and we want to reduce the risk of harm," a State Department spokesperson said.

The move elicited a warning from the U.S. Congress. House Speaker Nancy Pelosi said Wednesday that the U.S. must avoid war with Iran, and she declared the White House has "no business" moving toward a Middle East confrontation without approval from Congress.

Pelosi warned that the administration cannot rely on the last use of force authorization approved by Congress nearly 20 years ago for the wars in Iraq and Afghanistan. Since last week, House leaders have been asking for a classified briefing for lawmakers on the situation with Iran, but Pelosi said the administration indicated it couldn't come together "that fast."

U.S. President Donald Trump and Speaker of the House Nancy Pelosi attend the 38th Annual National Peace Officers' Memorial Service at the U.S. Capitol Wednesday. Pelosi is warning the White House has 'no business' moving toward a Middle East confrontation without approval from Congress. (Evan Vucci/Associated Press)

"The very idea that they would say that they would use the authorization of the use of military force that is 18 years old is not appropriate in terms of its scope, its geography, its timing for any actions they might take," Pelosi said.

The top Democrat on the Senate foreign relations committee also warned the administration against any action without approval from lawmakers.

"Congress has not authorized war with Iran, and the administration, if it were contemplating military action with Iran, must come to Congress to seek approval," said Sen. Bob Menendez in a statement at a committee hearing.

Democratic Sen. Tom Udall of New Mexico is seeking support for a bill he put forward to try to stop the Trump administration from leading the U.S. into an "unconstitutional, endless" war.

This makes it as clear as day: The Trump administration is breaking w/our allies & attempting to provoke war with Iran. I’m taking action to prevent it.<br><br>Only Congress has the authority to declare war. My bill blocks <a href="https://twitter.com/POTUS?ref_src=twsrc%5Etfw">@POTUS</a> from dragging us into an unconstitutional, endless war. <a href="https://t.co/6ly8ZzWexA">pic.twitter.com/6ly8ZzWexA</a> —@SenatorTomUdall

'We have to be careful'

Also on Wednesday, Germany — which has 160 soldiers in Iraq — suspended military training operations, citing increasing regional tensions, while the Netherlands suspended a mission providing assistance to Iraqi local authorities, Dutch news agency ANP said.

Both the United States and Iran have said they do not want war, and Iraqi Prime Minister Adel Abdul Mahdi said on Tuesday he had indications "things will end well" despite the rhetoric.

Iraq has said it will keep strong ties with Iran, but also with the United States and regional neighbours, some of whom, like Saudi Arabia, consider Tehran an archrival.

"I think we are now in a quite dangerous situation where a miscalculation by either side could lead us into conflict," U.S. Senator Chris Coons, a Democrat on the Senate foreign relations committee, told CNN in an interview on Wednesday.

"When you project force into a very volatile region and you've got real tension between Iran and the Saudis — we have to be careful. We need a strategy," Coons said, echoing a call by Congress for the government to brief lawmakers.

The State Department reissued a travel advisory for Iraq that said U.S. citizens were at high risk of violence and kidnapping.

"Anti-U.S. sectarian militias may also threaten U.S. citizens and Western companies throughout Iraq," it said.

A senior Iranian official said on Wednesday that any conflict in the region will have "unimaginable consequences."

Iranian leader threatens uranium enrichment

Iran's supreme leader issued a veiled threat in the same speech in which he stated that "no one is seeking war," saying it wouldn't be difficult for the Islamic Republic to enrich uranium to weapons-grade levels amid rising tensions with the U.S., state media reported Wednesday.

Speaking Tuesday night in Tehran at an iftar, the traditional dinner Muslims have when breaking their daylong fast during the holy month of Ramadan, Ayatollah Ali Khamenei's reported comments first focused on him downplaying the chances of a wider conflict in the Mideast with America.

Iranian Supreme Leader Ayatollah Ali Khamenei, seen in 2009, reportedly said of the U.S. that 'neither we, nor them is seeking war. They know that it is not to their benefit.' (Caren Firouz/Reuters)

He reportedly told senior officials that his country won't negotiate with the United States, calling such talks "poison." But he also said, "Neither we, nor them is seeking war. They know that it is not to their benefit."

In Wednesday's edition, the state-run IRAN newspaper carried his comments on the nuclear program, his first since Iran announced it would begin backing away from the accord itself.

Tehran is threatening to resume higher enrichment in 60 days if no new nuclear deal is in place, beyond the 3.67 per cent permitted by the current deal between Tehran and world powers.

Iranian officials have said that they could reach 20 per cent enrichment within four days. Though Iran maintains its nuclear program is for peaceful purposes, scientists say the time needed to reach the 90 per cent threshold for weapons-grade uranium is halved once uranium is enriched to around 20 per cent.

"Achieving 20 per cent enrichment is the most difficult part," Khamenei said, according to the newspaper. "The next steps are easier than this step."

It was a telling remark from the supreme leader. Iran is not known to have enriched beyond 20 per cent previously and it's unclear how far Tehran is willing to go in this process.