The head of a powerful Iraqi militia wants a formal role for Shi'ite paramilitaries in securing the border with Syria, a move that could deepen U.S.worries about Iran's growing sway over a strategic corridor of territory from Tehran to Beirut.

Iraq's Shi'ite militias, many of which are supported by Iran and oppose the presence of U.S. troops in the region, have sent reinforcements to the frontier after fighting flared between U.S.-backed Kurdish forces and Islamic State militants on the Syrian side.

Qais al-Khazali, the leader of Iranian-backed Asaib Ahl al-Haq, urged the government to provide a more formal, long-term border protection role for the militias.

"Securing Iraq's borders with Syria is among the most important duties of the Popular Mobilization Forces right now," he said in an interview with Reuters at his office in the Shi'ite holy city of Najaf on Saturday.

"The Daesh [IS] threat against Iraq won't end as long as Syria is unstable. The PMF proved it is the military side most capable of dealing with Daesh... maybe the armed forces can invest the PMF in duties that include border security," Khazali said.

Asaib Ahl al-Haq Shi'ite militia fighters from the FILE - Asaib Ahl al-Haq Shi'ite militia fighters from the south of Iraq run during a mission to take control of Sulaiman Pek village from Islamist State militants, in the northwest of Tikrit city, Sept. 1, 2014. FILE - Asaib Ahl al-Haq Shi'ite militia fighters from the south of Iraq run during a mission to take control of Sulaiman Pek village from Islamist State militants, in the northwest of Tikrit city, Sept. 1, 2014.

Asaib Ahl al-Haq is part of the PMF, an umbrella grouping of mostly Iran-backed and trained Shi'ite paramilitary groups. The PMF was made formally part of the security forces this year after helping the military defeat Islamic State in Iraq in 2017.

It remains separate from the military and police, however, raising questions over whom the militias will answer to and what their exact role will be if they are fully integrated into Iraq's security structure.

Khazali said paramilitary commanders should retain leadership positions and that "the government needs to provide bases and weapons depots."

The growing presence of Iran-backed militias on the frontier has caused tensions with Washington, which has special forces on the Syrian side to back Kurdish-led fighters battling IS.

Shi'ite Popular Mobilization Forces (PMF) fighter FILE - Shi'ite Popular Mobilization Forces (PMF) fighters remove an Islamic State flag after liberating the city of Al-Qaim, Iraq, Nov. 3, 2017. FILE - Shi'ite Popular Mobilization Forces (PMF) fighters remove an Islamic State flag after liberating the city of Al-Qaim, Iraq, Nov. 3, 2017.

A formal PMF border role would exacerbate that friction as Washington seeks to counter Iran's sway over territory stretching from Tehran to the Mediterranean via Iraq and Syria.

Iran's allies in that territory include Iraqi and Lebanese fighters and politicians, and Syria's President Bashar al-Assad.

The risks of having Shi'ite fighters and U.S. forces in close proximity were laid bare in July when the PMF vowed to "not be quiet" over an alleged U.S. air strike it said killed 22 of its members inside Syria.

The United States denied involvement in the strike.

The Iraqi military, which Washington supports, is deployed along the frontier, but PMF leaders have said they are taking the lead in securing it, including around the town of al-Qaim which borders Syria's Deir al-Zor province.

"The border was not secure before. Our operations have fixed that completely," a senior PMF commander said in October.

Iraq's military relied on the PMF support to defeat IS. It says the militias are now crucial to securing the sprawling Syrian border.

Iraqi Sunni and Kurdish politicians have called for disarming the PMF. They say the militias are responsible for widespread abuses including extra-judicial killings and displacing non-Shi'ite populations, and in effect report to Tehran, not the government in Baghdad.

The PMF says any abuses were isolated incidents and not systematic and that those who committed them have been punished.

US troop presence 'unacceptable'

The PMF, estimated at 150,000 members, includes groups which fought the U.S. military after the 2003 invasion that toppled Saddam Hussein, and individuals against whom Washington has imposed Iran-related sanctions.

Members of Congress have sought sanctions against Khazali's group. Khazali denied it is currently receiving support from Iran.

"We don't expect a good future for relations between Iraq and the United States under [President Donald] Trump," he said, and reiterated his call for U.S. troops to leave Iraq.

"A [U.S.] training role is one thing but presence of combat forces is unacceptable. Parliament should oppose this. Daesh is no longer a military threat, so there should be a reduction" in U.S. troops, he said.

The Pentagon says over 5,000 troops are deployed in Iraq.

Khazali's militia started as a splinter group of the Mahdi Army, a force formed by anti-American Shi'ite leader Moqtada al-Sadr in the U.S. occupation. Under his leadership, it gained notoriety for its attacks against U.S. forces.

He and Hadi al-Amiri, the veteran leader of the Badr Organisation who contested Iraq's May general election, were among the first to announce late last year they were putting their paramilitaries under the orders of the prime minister.

Iran has provided training and weapons to both groups.