Pentagon spokesman Army Colonel Rob Manning, who announced the new number, said that troop numbers are now declining in Syria.
The United States now has approximately 2,000 troops on the ground in Syria, where they have been helping train and advise partner forces in the fight against ISIL.
The Pentagon has officially announced that there are now 2,000 troops in Syria - a fourfold increase from the previous figures given.
In Syria, under Obama administration accounting practices known as Force Management Levels, the Pentagon only acknowledged 503 troops in Syria, though officials privately admitted there were significantly more forces there.
About 2,000 American troops are in Syria fighting the Islamic State, a Pentagon spokesman said on Wednesday, nearly four times the total previously disclosed as the Trump administration changes how troop numbers are publicly counted.
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Russia's military said on Thursday it had accomplished its mission of defeating Islamic State in Syria, and today there are no remaining settlements in the country under Islamic State control. USA forces in Syria and Iraq are "trending downward", he added, without specifying whether the 2,000 troops in Syria included the 400 Marine artillerymen that the Pentagon announced last week would soon leave the country.
"We are going to maintain our commitment on the ground as long as we need to, to support our partners and prevent the return of terrorist groups", Pentagon spokesman Eric Pahon said.
Russia's military deployed in Syria will now focus on preserving ceasefires and restoring peaceful life, he said.
"Important work remains to ensure lasting defeat" of ISIS, he said.
ISIL's self-proclaimed caliphate has crumbled this year in Syria and Iraq, with the group losing the cities of Mosul, Raqqa and swathes of other territory.