US-Iraq relations have grown tense once again, after a series of ups and downs over the years, from the 1990 Gulf war though crippling sanctions to the 2003 invasion that toppled Saddam Hussein and the fight against the Islamic State group.

But a year after Iraq declared victory over IS following a three-year war against the jihadists in which it was also backed by Iran, the Americans are seen by some as an unwanted "occupying force".

And if they do stay, "every Iraqi will have the legitimate right to confront them by any means," warned Mohammed Mohie, spokesman for the Hezbollah Brigades in Iraq, a force close to Iran that has also fought on the side of President Bashar al-Assad in Syria.

the powerful leader of the Asaib Ahel al-Haq (League of the Righteous) armed group, Qais al-Khazali, echoed the warning.

"If we are ever needed, we are ready," he said.

- Americans 'very worried' -

There were nearly 4,500 US troops killed in Iraq between 2003 and 2011, including in fighting with Shiite armed groups.

But before any decision to take up arms again and spill more blood, Mohie said he wants to give lawmakers a chance to set a timeframe for the departure of US troops from Iraq.

A bill has been tabled in parliament, and there could be a rare show of unanimity in support of it between its two biggest factions: populist cleric Moqtada Sadr's alliance, which champions Iraq's independence, and the pro-Iranian bloc of former anti-IS fighters.

"For three years, the main rivalry in parliament has been among Shiite factions," said Renad Mansour, a researcher at the Chatham House think-tank.

"They cannot agree on the choice of a minister, but they do on one point: that the experience of having America in Iraq has been bad."

Ironically, the person who has given new impetus to the proposed timetable for American troops to leave Iraq is the US president himself.

At the weekend, Trump provoked indignation even among Washington's allies in Baghdad when he said he plans to keep American forces in Iraq to keep an eye on Iran.

As a result, US diplomats and military officials in Baghdad were "very worried" and doing everything to "minimise" the impact of the remarks, said Mansour.

Trump had already irritated the Iraqis by not meeting any of the country's officials during a surprise Christmas visit to US troops stationed less than 200 kilometres (125 miles) from Baghdad.

- Step by step -

US forces left Iraq in 2011, only to return in 2014, at the head of the coalition against IS in Iraq and Syria.

But the United States is now seeking to use Iraq as "a base for attacking neighbouring countries," Khazali told AFP.

"Trump does not understand that Iraq is now a strong country. But he can be sure that if he persists, he will pay very dearly," said the Asaib leader wearing a Shiite white turban.

Mohie said adoption of the bill on a US withdrawal would be the "first step".

But he swiftly added that "we think the United States will again challenge the popular will" by trying to stay in Iraq.

In that case, Mohie said his forces and others like it would move to the "second step" and take up arms against "an occupying force".

"The resistance factions have gained capabilities and expertise in the fight against IS," he said.

The experience they gained "will serve to confront any army that threatens Iraq and its sovereignty."

He said that above all any confrontation would allow Shiite factions "to find an external threat on which to focus attention rather than their internal problems."