The top US diplomat's unannounced visit marked an effort to stand up Washington's ties with Baghdad as it pushes ahead with its "maximum pressure" against Tehran -- a US arch-rival, but an ally of Iraq.

Pompeo abruptly cancelled talks in Germany and made a lengthy detour from a European tour to spend four hours in Iraq, where he met both President Barham Saleh and Prime Minister Adel Abdel Mahdi.

"We talked to them about the importance of Iraq ensuring that it's able to adequately protect Americans in their country," Pompeo told reporters after the meetings.

"They both provided assurances that they understood that was their responsibility," he said.

Pompeo said he made the trip because Iranian forces are "escalating their activity" and said the threat of attacks were "very specific".

"These were attacks that were imminent," Pompeo said.

He declined to go into further detail on the alleged plot, which has been met with scepticism in numerous quarters, with leading Democratic lawmakers fearing that President Donald Trump's administration is seeking to spark a war with Iran.

In the latest US move, the Pentagon said it was sending several massive, nuclear-capable B-52s to the region.

The deployment was in response to "recent and clear indications that Iranian and Iranian proxy forces were making preparations to possibly attack US forces," the Pentagon said.

On Sunday, Washington announced it was dispatching an aircraft carrier strike group to the Middle East as national security advisor John Bolton warned Iran that Washington would respond with "unrelenting force" to any attack by Tehran, including by its regional allies.

Iraq's majority Shiite population enjoys religious kinship with Iran, which played a significant role in helping Baghdad to fight the Islamic State extremist movement.

President Hassan Rouhani paid an official visit to Iraq in March, where he denounced pressure from the "aggressor" United States, which deposed Iran's arch-enemy Saddam Hussein in a 2003 invasion.

Pompeo, whose trip to Iraq is his second this year, said he spoke at length about Iran's influence with Shiite militias.

"We've urged the Iraqi government for its own security to get all of those forces under Iraqi central control," Pompeo said.

"In each of those meetings, those two leaders promised that that was their objective, too, they were moving towards that goal," he said.

In Iraq, a debate has been raging in recent months over the fate of some 5,200 US troops stationed across the country.

Their presence angers the Hashed al-Shaabi, a paramilitary force that is dominated by pro-Iran factions which played a key role alongside government forces in the fight against IS.

In a press conference a few hours before Pompeo's arrival, Abdel Mahdi said Iraq would not accept any attack on foreign troops on its land.

"Iraq really is taking the responsibility to avoid any attack on any of our friends here, coalition forces or any of our friends here," he told reporters.

"This is an obligation that Iraq would honour, (and) not accept any attack on anyone -- whether Iraqi, foreigner, whether it's an embassy or a company or a military mission," he said.