A small contingent was pictured last week patrolling the wells in Deir Ezzor.
Mr Trump had announced the US was withdrawing all troops early last month, paving the way for a Turkish offensive on Kurdish forces in north-east Syria.
He later declared in a speech that the US was done fighting “endless wars” in the region’s “blood-soaked sands”.
However, in the face of pressure the president appears to have ridden back on his promise.
Experts say the return of troops is unlikely to only concern oil, as Mr Trump has stated. The output from the fields is limited, with little export opportunity for the US.
Reports suggest national security hawks seized on the oil as a way of capturing Mr Trump’s attention and convincing him to keep some US forces in Syria to help support their Kurdish partners as well as for leverage.
The oil fields are currently under the control of fighters with the US-backed Syrian Democratic Forces (SDF), however the Bashar al-Assad government would like to see it return under its control.
Since discovery of oil in MidEast, many in region said: U.S. only here to steal our oil.
U.S. denied, claimed it's about democracy, human rights, women etc.
Not sure if Americans realize but these pictures of U.S. troops in northeast Syria are HUGELY damaging to U.S. image. pic.twitter.com/C3exwUbHtD
— Jenan Moussa (@jenanmoussa) November 2, 2019
US officials said the order approved by Mr Trump does not include any mandate for the US to take Syria’s oil, despite the president declaring multiple times that the US is "keeping the oil."
The White House and the Pentagon have so far been unable to explain what he means by that. Mark Esper, Defense Secretary, said Friday he "interprets" Mr Trump’s remarks to mean the military should deny Islamic State access to the oil fields.
The legal authority for US troops going into Syria to fight Isil militants was based on the 2001 and 2002 Authorisations for Use of Military Force that said US troops can use all necessary force against those involved in the September 11 attacks on America and to prevent any future acts of international terrorism.
Senator Tim Kaine, a Virginia Democrat, called the mission misguided. "Risking the lives of our troops to guard oil rigs in eastern Syria is not only reckless, it’s not legally authorized," Mr Kaine told The Associated Press.
"President Trump betrayed our Kurdish allies that have fought alongside American soldiers in the fight to secure a future without ISIS – and instead moved our troops to protect oil rigs."