All US troops ordered out of northern Syria

All 1,000 US troops based in northern Syria have been ordered to leave the country, a US official said Monday, after Turkey-led offensive began last week.

"We are executing the order," a US official told AFP.

It comes one day after President Donald Trump ordered American troops to evacuate northern Syria, as fighting between Kurdish-dominated Syrian Democratic Forces (SDF) and Turkish-backed rebels continues.

The US will leave a small contingent of 150 soldiers in the southern Syria base at Al-Tanf, the official said.

A US official told AFP Washington is "executing the order" to withdraw, but did not provide additional details.

Turkey launched an offensive into northern Syria on Wednesday, looking to carve out a "safe zone" in Kurdish-controlled areas.

The Syrian Democratic Forces (SDF) have so far failed to hold back the Turkish-backed offensive, with key border towns taken by Syrian fighters.

Trump on Sunday defended his decision to pull American troops away from the border in a tweet.

"Very smart not to be involved in the intense fighting along the Turkish Border, for a change," he said.

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

Kurdish authorities said their "abandonment" by the US has forced them to hand over territory to the Syrian regime to prevent them being taken by the Turkish force.

Defence Secretary Mark Esper said the move to retreat came after the US learned that Turkey was pressing further into Syria than had been expected.

Trump warned of new sanctions on Turkey over the offensive.