Planning is underway for a "full" and "rapid" withdrawal of US troops from Syria, a US defense official told CNN on Wednesday.

The decision, which would be a reversal from previously stated US policy, was made by President Donald Trump, who has long signalled his desire to get out of Syria, the official added.

On Wednesday morning, the President tweeted, “we have defeated ISIS in Syria, my only reason for being there during the Trump Presidency.”

Even though the US will continue to maintain troops in Iraq with the capability of launching strikes into Syria, a US withdrawal of ground forces would fulfil a major goal of Syria, Iran and Russia and risks diminishing US influence in the region.

The US has about 2,000 troops on the ground in the country, where they are primarily training local forces to combat ISIS. The US-backed and Kurdish-led Syrian Democratic Forces have had some recent success against the terror group and are on the cusp of capturing the last major town held by ISIS east of the Euphrates.

Estimates vary as to how many ISIS fighters are left in Syria. In the town of Hajin, the terror group's last redoubt, the coalition estimated some 2,000 ISIS fighters were present. But a Defense Department inspector general report put the number of ISIS members in Syria and Iraq as high as 30,000.

The US has forces in Iraq ready to launch attacks in Syria if necessary. In the last few weeks, the US-led coalition fighting ISIS has carried out hundreds of air and artillery strikes targeting ISIS in Syria. Some of those strikes were launched from neighbouring Iraq, where the US has over 5,000 troops. Hundreds of US troops have also been training local forces at At Tanf in southern Syria, where Russia-backed pro-regime forces are seeking to oust the US presence.

Pentagon spokesman Col. Rob Manning told CNN that, “at this time, we continue to work by, with and through our partners in the region.”

The US-led coalition recently denied any change to the US presence in Syria.

“Any reports indicating a change in the US position with respect” to the US military presence in Syria “is false and designed to sow confusion and chaos,” the coalition said in a statement earlier this month.

Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff, said this month that the US needed to train thousands of local fighters to ensure a lasting defeat of the terror group.

US forces were recently directed by Secretary of Defense James Mattis to establish observation posts along the Syria-Turkish border as part of an effort to reduce tensions between Turkey and America's Kurdish allies in the fight against ISIS.

News of the planned withdrawal was earlier reported by The Wall Street Journal.