The Pentagon on Thursday said it has 2,000 US troops in Syria, more than twice the 900 it has previously said it has, and that the additional troops are considered temporary forces that were sent to support the mission against Islamic State militants.

Pentagon spokesperson Major General Pat Ryder told reporters he did not know how long the number had been 2,000, but it was probably months at a minimum and pre-dated the fall of Syrian President Bashar al-Assad.

“I learned the number today ... as somebody who’s been standing up here telling you 900, I wanted to get you what we had on that,” Ryder said.

Asked by reporters how long the number of troops had been at 2,000, Ryder did not provide a specific length of time but said: “I think it would probably be fair to say, at a minimum, months... it’s been going on for a while.”

The US had said publicly for several years that it had 900 troops in Syria who were working with local forces to prevent a resurgence of Islamic State, which in 2014 seized large swathes of Iraq and Syria but was later pushed back.

President Joe Biden’s administration has said that US troops will be staying in Syria, though President-elect Donald Trump could remove them when he takes office on January 20.

During his first administration, Trump attempted to remove US troops from Syria but had been met with resistance from officials and ultimately some troops remained.

Earlier this month, Syrian rebels toppled Assad’s government and seized control of the capital Damascus. The US military has continued carrying out strikes against Islamic State militants.

This is not the first time in recent years that the Pentagon has had to revise the number of troops it has in a country. In 2017, the US military disclosed that it had 11,000 US troops in Afghanistan, thousands more than it had previously disclosed.