The Trump administration is planning to pull all US troops out of Syria, a defence official said on Wednesday, as President Donald Trump declared victory against the Islamic State.

The president, in a message on Twitter, said the United States had "defeated Isis in Syria, my only reason for being there during the Trump Presidency."

We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency.

— Donald J. Trump (@realDonaldTrump) December 19, 2018

His statement came shortly after news organisations reported that the White House had made a decision on Tuesday to abruptly remove the entire US force of more than 2,000 troops from Syria and end the extended American ground campaign against the Islamic State.

Trump has long promised to conclude the campaign against the Islamic State, also known as Isis, and has questioned the value of costly and dangerous military missions overseas. But US troops, working alongside Syrian partner forces, have struggled to eradicate remaining pockets of militants in central Syria. An abrupt American withdrawal would raise questions about whether the militants would be more easily able to regain strength.

The decision is the latest twist in American leaders' unsuccessful quest to craft a solution for Syria's long civil conflict, which has drawn in US allies and adversaries including Turkey, Russia and Iran.

Both the Trump and Obama administrations have resisted becoming more involved in Syria's larger civil war but many senior officials - including at the State Department and Pentagon - have supported an ongoing troop presence in Syria until security conditions improve and a political solution can be reached.

Defence officials, speaking on condition of anonymity to discuss a decision that has not yet been announced, said the withdrawal was expected to occur as quickly as possible and would affect the entire force of more than 2,000 US service members. The Wall Street Journal first reported on Wednesday that US troops would be removed from northeast Syria.

US forces are mostly in north central and northeast Syria, but they also have a smaller ground presence in the southeast Syria along the border with Jordan.

The unexpected White House move comes as tensions increase sharply with NATO ally Turkey, which has promised to launch a military offensive against the US partner forces in Syria, which Ankara considers part of a Kurdish terrorist group. Losing their US ground partner will be a major blow to the Syrian Kurdish forces.