US will impose tough sanctions on Turkey: Trump

President Donald Trump has said the US planned to impose tough sanctions on Turkey following its into Syria in the wake of the withdrawal of US troops from the area.

"Dealing with @LindseyGrahamSC and many members of Congress, including Democrats, about imposing powerful Sanctions on Turkey. Treasury is ready to go, additional legislation may be sought. There is great consensus on this. Turkey has asked that it not be done. Stay tuned!" Trump said on Sunday in a Twitter post, Efe news reported.

The president's tweet about sanctions came after Defence Secretary Mark Esper insinuated that Turkish forces may have committed war crimes in Syria.

Esper said on the CBS programme "Face the Nation" that Turkey "appears to be" committing war crimes, including executing groups of captured fighters, in its offensive against Kurdish forces.

"It's terrible. It's a terrible situation. We condemn it. We have condemned it. It's- these are justice things that we told the Turks would happen and play out. Who's conducting it, it's unclear at this point and time. There are Kurdish regular for- I'm sorry there are Turkish regular forces and there are Turkish proxy forces," Esper said.

"Let's be clear, the US is not allowing this. Turkey committed this action. They decide to make an incursion in northern Syria despite our protestations, our urgings not to do it, all of our warnings. They decided to do it," Esper said.

The defence secretary said Trump ordered the withdrawal of all US troops from northern Syria.

Esper said the US planned to withdraw about 1,000 troops deployed in northern Syria, adding that the fighting between Turkey and the Kurds "has its roots in conflict that goes back 200 years".

Trump had made a similar observation about the conflict earlier in a tweet.

"The Kurds and Turkey have been fighting for many years. Turkey considers the PKK the worst terrorists of all. Others may want to come in and fight for one side or the other. Let them! We are monitoring the situation closely. Endless Wars!" Trump said.

Earlier this month, Turkish President Recep Tayyip Erdogan said he would send his forces into Syria, targeting the Syrian Kurdish militias east of the Euphrates River.