"It's very hard to beat a force where they have planes and they don't," Trump said, referring to Turkey and the Kurds during a speech in Washington.

"I hope they do [back up]," Trump said of the Kurds.

Western allies have criticised Turkey's operation to take control of Kurdish-controlled territory along the border, which it launched on Wednesday, saying it risks sparking a severe humanitarian crisis as well as reviving the Islamic State extremist group in the area.

Trump defended his decision to withdraw troops from Syria's border with Turkey on Saturday.

"I dont think our solders should be there for the next 50 years, guarding a border between Turkey and Syria when we can't guard our own borders at home," he said referring to the US border with Mexico.

The US has been allied with Syrian Kurds since 2014 in the war against Islamic State, and together they worked to defeat the territorial caliphate. The Kurdish-led force lost some 10,000 people in the war. Turkey has a long-running conflict with its own Kurdish minority.

"We killed ISIS we defeated, we did our job, we have to go home," he added.

Earlier on Saturday former US Defence Secretary Jim Mattis directly contradicted Trump saying the Islamic State is not defeated and will resurge if the US does not continue its war against the group.

"ISIS is not defeated we have got to keep the pressure on ISIS so they don't recover," Mattis said in an interview with NBC's Meet the Press. "In this case if we don't keep the pressure on then ISIS will resurge. It's absolutely a given that they will comeback."

Mattis resigned in December after Trump said he would withdraw troops from the country. The US kept some troops in the country after the announcement sparked international and domestic condemnation.

Australian Associated Press