The United States military on Wednesday announced it will reduce its presence in Iraq from 5,200 to 3,000 troops this month, formalising a move that had been long expected.

"We are continuing to expand on our partner capacity programmes that enable Iraqi forces and allow us to reduce our footprint in Iraq," Marine General Frank McKenzie, the head of US Central Command, said during a visit to Iraq.

The US and Iraq in June affirmed their commitment to the reduction of American troops in the country in coming months, with no plans by Washington to maintain permanent bases or a permanent military presence.

The US has about 5,200 troops that were deployed in Iraq to fight the armed group ISIL (ISIS). Officials in the US-led coalition said Iraqi forces are now mostly able to handle the ISIL remnants on their own.

Late Tuesday, a senior Trump administration official, speaking on condition of anonymity, told reporters on board Air Force One that such an announcement was coming and an announcement on the withdrawal of additional troops from Afghanistan also could be expected in the coming days.

'Continued commitment'

The US invaded Iraq in 2003 and left in 2011, but returned in 2014 after ISIL overran large parts of the country.

"In recognition of the great progress the Iraqi forces have made and in consultation and coordination with the government of Iraq and our coalition partners, the United States has decided to reduce our troop presence in Iraq from about 5,200 to 3,000 troops during the month of September," McKenzie said, according to an excerpt of his remarks provided by his office.

The remaining US forces will continue advising and assisting Iraqi security forces as they attempt to root out ISIL fighters, McKenzie said.

"The US decision is a clear demonstration of our continued commitment to the ultimate goal, which is an Iraqi security force that is capable of preventing an ISIS resurgence and of securing Iraq's sovereignty without external assistance," McKenzie said.

"The journey has been difficult, the sacrifice has been great, but the progress has been significant."

'Endless wars'

In 2016, Trump campaigned on ending the US's "endless wars" but US troops remain in countries such as Afghanistan, Iraq and Syria, albeit in smaller numbers.

"We kept America out of new wars and we're bringing our troops back home, we're bringing them back home from all these faraway places," Trump said in a campaign speech on Tuesday. "We've spent hundreds of billions of dollars, and what do we get out of it?"

Last month, during a meeting with the Iraqi prime minister, Trump redoubled his promise to withdraw the US troops still in Iraq.

Iraq's parliament voted earlier this year for the departure of foreign troops from Iraq, and US and other coalition troops have been leaving as part of a withdrawal.

Trump's meeting with the Iraqi Prime Minister Mustafa al-Kadhimi came amid a new spike in tensions between Washington and Tehran after Washington said it would seek to reinstate all previously suspended US sanctions on neighbouring Iran at the United Nations. Iraq and Iran have close political, economic, and military ties.

Fears of open conflict between the US and Iran grew in January after an American drone strike near Baghdad's airport killed top Iranian General Qassem Soleimani and Iraqi militia leader Abu Mahdi al-Muhandis.

Angry Iraqi legislators, spurred on by Shia political factions, passed a non-binding resolution to remove all US-led coalition forces from the country.

In response to the Soleimani killing, Iran, on January 8, launched a ballistic missile attack on al-Asad airbase in Iraq, which resulted in traumatic brain injuries to more than 100 American troops.

Afghanistan next?

Trump has also pushed hard to pull back US forces from Afghanistan, where they rose to more than 12,000 under his watch to pressure the Taliban and ISIL.

The number fell to about 8,600 in July following the February peace accord with the Taliban and McKenzie has said they could all be gone by May 2021 if the Taliban and the Afghan government reach their own agreement.

But Trump's pressure on the Pentagon to more quickly disengage in the Middle East and Afghanistan has strained relations between the White House and US defence chiefs.

Former defence secretary James Mattis quit in December 2018 after Trump declared all US troops would leave Syria.

Under Mattis' successor Mark Esper, the Pentagon has remained wary of hasty withdrawals, cautious that the Taliban would overwhelm Afghan government forces if the US pulls out too quickly.

It has also taken account of Iran's influence in Iraq and the Middle East, which could grow if American forces vacate the region.