WASHINGTON - US forces must remain in Iraq so the United States can keep a close eye on neighbouring Iran, according to President Donald Trump.

In an interview with CBS' Face the Nation programme, Trump highlighted the importance of a key military base in Iraq that he said was crucial to the surveillance of the Islamic Republic's activities.

"We spent a fortune on building this incredible base, we might as well keep it," he said in an apparent reference to Ain al-Asad airbase in western Iraq, which he visited during a trip to the country in December.

"And one of the reasons I want to keep it is because I want to be looking a little bit at Iran because Iran is a real problem," Trump said in the interview broadcast on Sunday.

"We were in many, many locations in the Middle East in huge difficulty. Every single one of them was caused by the number one terrorist nation in the world, which is Iran."

When asked whether he planned to use US forces in Iraq to "strike" Iran, Trump responded: "No … all I want to do is be able to watch."

Trump also indicated the US military installation would be useful for monitoring developments in the wider Middle East.(FA)