On a lightning trip to Iraq that came after mounting criticism that he had not already visited US forces abroad, Trump said he was concerned about making the journey "when I heard what you had to go through."

"I had concerns about the institution of the presidency. Not for myself personally. I had concerns for the First Lady, I will tell you," Trump, who made the trip along with his wife Melania, told reporters.

Trump, who left unannounced from Washington in the dead of night in a darkened plane, said that a "couple" of trips had been previously set up but canceled as word got out.

"Pretty sad when you spend $7 trillion in the Middle East, and going in has to be under this massive cover with planes all over and all of the greatest equipment in the world, and you do everything to get in safely," he said.

For "security reasons," the White House was only able to invite Iraqi Prime Minister Adel Abdel Mahdi to a meeting with Trump two hours before the scheduled time, spokeswoman Sarah Sanders said.

Abdel Mahdi was in another part of Iraq and could not attend, she said.

Trump's trip came days after his sudden decision to withdraw all 2,000 US troops from Syria, arguing that the Islamic State jihadist group had been defeated and that the United States was paying too much on foreign interventions.