"The Iraqi constitution rejects the use of Iraq as a base for hitting or attacking a neighboring country", he said.
"We have to protect other things that we have", Trump told CBS on Sunday, but said the troops will be "coming back in a matter of time".
Asked if that meant he wanted to be able to strike against Iran, Trump said, "No, because I want to be able to watch Iran".
The US could rely heavily on intelligence work in Afghanistan, he said, and respond to developments in Syria from US bases in neighbouring Iraq.
Trump said the United States had spent " a fortune on the Al-Asad Air Base in western Iraq, and added: "We might as well keep it.
During a surprise visit to Iraq late previous year, Trump made clear that he has "no plans at all" to remove U.S. troops from the Middle Eastern country, stressing his interest in wanting more soldiers deployed there from Syria. "We can come back very quickly, and I'm not leaving", Trump said.
There is already concern that Trump's comments will encourage some Iraqi politicians, especially those with close ties to Iran, to push more strongly for the approximately 5,200 USA troops to leave Iraq.
Even as U.S. troops are pulling out of Syria, President Trump has said he wants to keep some forces in the region "to protect Israel" and "watch Iran".
59-Fight MMA Veteran with Pending Murder Charges Escapes from Prison Transport
Last month, Marks' wife, Ginell McDonough, was arrested in Muskegon County, Michigan and charged with harboring a fugitive. Marks was arrested in MI last month on a charge alleging he broke into Scott's home in Temple, Texas, in August.
"I don't know how many times you can say this but I prefer the president would stay off Twitter - particularly with regard to these important national security issues", Sen. "Trump did not ask us to keep USA troops to watch Iran".
He also said the USA isn't really leaving because they will maintain the base in Iraq - "a fantastic edifice".
He blamed the intelligence services for America's involvement in Iraq. Left unsaid is that these grievances are a product of upheaval caused by the 2003 U.S. invasion of Iraq and the USA support for sectarian rebels in Syria starting in 2011.
Saleh said United States forces were in the country legally under an agreement between the two countries, but that "any action taken outside this framework is unacceptable".
The US "deep state" is firmly against withdrawal from Syria, Shoebridge noted.
The BBC's Paul Adams says this poses a problem for the government in Baghdad and could complicate delicate negotiations over United States use of the Al Asad Airbase.
Now, after defeating IS militants in their last urban bastions, Iraqi politicians and militia leaders are increasingly speaking out against the continued presence of US forces on Iraqi soil.
Trump also said he wants to leave troops in Iraq, however, in order to keep an eye on the regime in Iran.
Iraq announced the fight against ISIS was over on December 9, 2017, after the group seized the country's second-largest city Mosul, as well as one-third of the rest of the country, before being toppled by an Iraqi military campaign that was backed by the USA -led coalition.
The United States has pressured Iraq to wean itself off Iranian natural gas, which generates almost half of Iraq's electricity, with promises of USA investment in the country's crumbling infrastructure.