Turkey on Monday vowed to continue fighting a US-backed Kurdish militia which it views as a terrorist group after Donald Trump warned of economic devastation if Ankara attacks Kurdish forces as American troops withdraw.

Turkish President Recep Tayyip Erdogan's spokesman Ibrahim Kalin said on Twitter that there was “no difference” between the Islamic State extremist group and the Kurdish People's Protection Units (YPG) militia.

“We will continue to fight against them all.” Turkey's response came after Trump on Sunday warned on Twitter: “Will devastate Turkey economically if they hit Kurds.” “Mr @realDonaldTrump Terrorists can't be your partners & allies.

Turkey expects the US to honour our strategic partnership and doesn't want it to be shadowed by terrorist propaganda,” Kalin said in a tweet to the US president.

Turkey views the YPG as a “terrorist offshoot” of the outlawed Kurdistan Workers' Party (PKK), which has been waging an insurgency against the Turkish state since 1984.

The PKK is blacklisted as a terrorist organisation by Ankara, the United States and the European Union. But Washington has been working closely in recent years with the YPG, providing military support and training, in the fight against IS in Syria.

Kalin said that it was “a fatal mistake to equate Syrian Kurds with the PKK”, saying that Turkey fought against terrorists not Syrian Kurds.

American support to the YPG has been one of the main sources of tension between Turkey and the US, but there appeared to be some improvement on the issue after Trump said last month 2,000 American troops would withdraw from Syria.

Ankara welcomed the pullout decision after Erdogan told Trump in a phone call last month that Turkey could finish off the last remnants of IS.

However, there has been growing friction between Turkey and the US over the fate of the YPG, especially after US Secretary of State Mike Pompeo previously said Washington would ensure Turkey would not “slaughter” Kurds.

And before a visit to Ankara last week, White House National Security adviser John Bolton said the US retreat was conditional on the safety of the Kurdish fighters, provoking angry retorts from Turkish officials.