Kurdish-led forces in Syria have welcomed a White House reversal of President Donald Trump's decision to pull out all US troops from the country.

With Washington's allies poised for victory against Islamic State fighters who are making a final stand in a pocket near the Iraqi border, the White House announced plans on Thursday to keep "a small peacekeeping force" in Syria.

The announcement partially reversed Trump's abrupt decision in December to withdraw the entire 2000-strong US contingent, which had alarmed Washington's Kurdish allies and prompted Defence Secretary Jim Mattis to quit.

An administration official on Friday put the total number of troops to stay at 400, split between a safe zone being negotiated for northeastern Syria and a US military base at Tanf, near the border with Iraq and Jordan.

Although the US contingent would now be small, the official indicated that the 200 troops in the northeast would be part of a wider commitment of about 800-1500 troops also drawn from European allies to set up and observe a safe zone.

Speaking to reporters in Washington, Trump insisted that the decision to leave some US troops did not constitute a reversal of his December announcement.

Kurdish leaders suggested the US troop decision could have a large impact on the fate of the area, preventing a security vacuum.

Washington could keep control of the air space and European allies could complement the force with more troops.

A planned assault on the final Islamic State redoubt in the area, Baghouz, would effectively end the territorial rule of the jihadist group, which ruled around a third of both Iraq and Syria at its self-proclaimed Caliphate's height four years ago.

Reporters near the front line at Baghouz saw dozens of trucks leaving loaded with civilians, and empty ones driving inside accompanied by fighters from the Syrian Democratic Forces (SDF) militia.

Mustafa Bali, an SDF spokesman, said the evacuation would be completed on Friday, with thousands of civilians still inside the pocket from an estimated 7000 at the start of the day, though by nightfall it was unclear if it had been completed.

The battle against Islamic State in the area has taken place since December in the shadow of Trump's abrupt decision to withdraw all US troops, which raised doubt about the future of the fighters that had served as US allies on the ground.

The Kurdish-led authorities in the north welcomed the White House reversal.

They had feared that a total US withdrawal would leave their area exposed to attack by Turkey, which sees the main Kurdish militia as a national security threat.

"This decision may encourage other European states, particularly our partners in the international coalition against terrorism, to keep forces in the region," Abdulkarim Omar, co-chair of foreign relations in the region held by the U.S.-backed SDF told Reuters.

"I believe that keeping a number of American troops and a larger number of (other) coalition troops, with air protection, will play a role in securing stability and protecting the region too."