The Pentagon said on Thursday that the US was not profiting from Syrian oil, despite sending troops to protect oilfields in the country's east.
"The revenue from this is not going to the US. This is going to the SDF," said Pentagon spokesman Jonathan Hoffman, referring to the Kurdish-led Syrian Democratic Forces, a key US ally in the military campaign against ISIS.
The goal is to allow the SDF to have "the ability to work on building up their strength on the de-ISIS campaign," Mr Hoffman said.
"So it's preventing ISIS from getting it, and allowing the Kurds and the SDF in there to have control of it as well."
When ISIS militants controlled the oilfields in 2015, they pumped 45,000 barrels a day worth $1.5 million (Dh5.5m), paying for domestic and overseas attacks, said Rear Adml William Byrne, vice director of the joint staff.
"We're just not going to let that happen again," Adml Byrne said.
Last year, Syria ranked 75th among oil-producing countries, the US Energy Information Administration said, with a daily output of 28,000 barrels.
US President Donald Trump announced on October 6 that about 1,000 US troops were leaving north-east Syria, where they had kept an uneasy peace between neighbouring Turkey and Kurdish fighters.
The withdrawal allowed Turkey to carry out a military operation against the Kurds, although Mr Trump later said a "small number" of US troops would stay to secure the oil.
He has long been a proponent of controlling Syria's oil fields.
At a gathering of police chiefs last week, Mr Trump repeated that he did not want the US policing the conflict between Turkey and the Kurds.
"But we're keeping the oil. Remember that," he said, to applause.
"I've always said that: 'Keep the oil.' We want to keep the oil. $45m a month? Keep the oil. We've secured the oil."
US Defence Secretary Mark Esper recently said the mission of American troops in Syria was to stop ISIS accessing oilfields and securing "resources that may allow them to strike within the region, to strike Europe, to strike the United States".
Russia has called the US presence at the oilfields "international banditry".