"There was no decision to leave Iraq. I do not know what this letter is ... We are trying to find out where it comes from, what it is. There was no decision to leave Iraq," said Esper.
Pompeo said he did not consider the Iraqi parliament voting to be the final, official decision of the Iraqi government.
"We are confident that the Iraqi people want the United States to continue to be there to fight terrorists. And we will continue to do everything we need to ensure the security of America," said Pompeo.
Earlier it was reported that the US-led coalition of ISIS was allegedly ready to leave Iraq within a few weeks.
It was not immediately clear if all roughly 5,000 U.S. troops would leave Iraq.
Several helicopters could be heard flying over Baghdad on Monday night. It was not immediately clear if this was a related development. The letter said coalition forces would be using helicopters to evacuate.
“In deference to the sovereignty of the Republic of Iraq, and as requested by the Iraqi Parliament and the Prime Minister, the coalition will be repositioning forces over the course of the coming days and weeks to prepare for onward movement,” read the letter.