The United States has no plans to pull out militarily from Iraq, Defense Secretary Mark Esper told Pentagon reporters on Monday, following reports of a US military letter about preparations for a withdrawal.

"There's been no decision whatsoever to leave Iraq," Esper said, when asked about the letter, adding there had also been no plans issued to prepare to leave.

"There's been no decision made to leave Iraq. Period," he told reporters.

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The letter, cited by news agencies and US media, was sent to Iraqi military by US Marine Corps Brigadier General William H Seely III, commanding general of Task Force Iraq, the US-led military coalition against ISIL.

A top US general told reporters that the letter was a poorly worded draft document meant to only underscore increased movement of forces.

"Poorly worded, implies withdrawal. That's not what's happening," US Army General Mark Milley, chairman of the Joint Chiefs of Staff, told a group of reporters, stressing there was no withdrawal being planned.

"This was a mistake from McKenzie," Milley told reporters, referring to US Central Command commander General Frank McKenzie.

"The long and short of it is: It's an honest mistake by people trying to do the right things in highly dynamic situations, etc. It should not have been sent," he said, adding that the unsigned document was a draft sent to get input from Iraqi officials.

"It (the draft letter) was sent over to some key Iraqi military guys in order to get things coordinated for air movements, etc. Then it went from that guy's hands to another guy's hands and then it went to your hands. Now, it's a kerfuffle."

Esper added the US was still committed to countering ISIL in Iraq, alongside US allies and partners.

The reports came a day after the Iraqi parliament urged the government to expel foreign troops following the US killing of Iranian general Qassem Soleimani in Baghdad.

Iraqi caretaker Prime Minister Adel Abdul Mahdi told the US ambassador to Baghdad on Monday that both countries needed to implement the resolution, the premier's office said in a statement.

The US military has said there are about 5,000 US troops in Iraq.