Trump has ordered US troops to withdraw from key positions along Syria's northern border.

US President Donald Trump suggested Thursday that Washington could mediate between Turkey and Kurdish groups after Ankara launched a military offensive against Kurds who until recently were backed by the United States.

"We have one of three choices: Send in thousands of troops and win Militarily, hit Turkey very hard Financially and with Sanctions, or mediate a deal between Turkey and the Kurds!" Trump tweeted.

This third option was the latest twist in the president's changing stance regarding the fate of Kurdish militias that until recently were fighting alongside US troops in Syria.

Trump has ordered US troops to withdraw from key positions along Syria's northern border, saying he does not want to stand in the way of a Turkish offensive against what Ankara considers to be a threat from the Kurds.

After coming under fierce criticism from his own party, Trump then said he intended to limit Turkey's offensive with the threat of economic sanctions.