Vance says the decision to withdraw an unspecified number of troops was made alongside North Atlantic Treaty Organisation and Canada's allies to ensure their safety and security. "Period", Esper told reporters at the Pentagon on Monday, in what appears to have been an off-camera briefing. "We will follow the laws of armed conflict", Esper said.
The Iraqi resolution specifically calls for an end to an agreement by which Washington sent troops to Iraq over four years ago to help fight the Islamic State group.
The US would be "repositioning forces in coming days and weeks", as written in the letter after Iraqi MPs had asked for them to leave.
The resolution is non-binding, but reflects the growing frustration in Baghdad with continued United States operations in Iraq long after the official defeat of IS was proclaimed.
The authenticity of the letter, which was addressed to the Iraqi Joint Operations Command and carried by Iraqi media, could not be immediately verified by the Iraqi military.
Explaining the confusion, Pentagon Joint Chiefs Chairman Mark Milley said the official USA letter informing Iraq that American troops would begin pulling out was "genuine" but not meant to be sent at this time.
The letter, which was quickly circulated online although not confirmed as authentic by on-the-record sources, was from U.S. Marine Corps Brig.
As the letter was signed by a U.S. official, it was not immediately clear whether it applied to forces from the 76 countries which make up the worldwide coalition.
He said the U.S. remains committed to the campaign to defeat the Islamic State group in Iraq and the region.
However, Pentagon Joint Chiefs Chairman Mark Milley said the letter was only a "draft".
"Poorly worded, implies withdrawal", said Gen. Milley. It did not give a timeline.
The International Zone is the formal name of Baghdad's heavily fortified Green Zone which houses government buildings and foreign missions.
The resolution passed by a special session of the Iraqi parliament said the government should cancel its request for assistance from a US-led coalition.
The parliament was reacting several days after a US airstrike took out Iranian General Qassem Soleimani in Baghdad.
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