Donald Trump has reportedly approved an expanded US military mission to secure an expanse of oil fields across eastern Syria.

The US president’s actions raise a number of legal questions about whether American troops can launch attacks against Syrian, Russian or other forces if they threaten the oil.

After meeting with defence leaders on Friday, Mr Trump decided to commit US troops to defending a large swath of land controlled by Syrian Kurdish fighters stretching from Deir Ezzor to al-Hassakeh, officials told the Associated Press.

The decision commits hundreds of US troops to Syria, despite Mr Trump’s promise to get the bulk of the more than 1,200 American soldiers out of the country.

It could also force lawyers in the Pentagon to craft orders for troops which could see them firing on Syrian government of Russian fighters if they try to take back the country’s oil facilities. 

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 authorised,” Mr Kaine said.

“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.”

The Pentagon has not disclosed how many forces will remain in Syria.

But officials speaking to the Associated Press suggested the total number could be at least 800 troops. 

Lawyers are trying to work out details of the military order, officials told the news agency, which would make clear how far troops will be able to go to keep the oil under the Kurds’ control.

They said the order approved by Mr Trump does not include any mandate for the US to take Syria’s oil.

Mr Trump has said multiple times the US is “keeping the oil,” but the White House and the Pentagon have so far been unable to explain what he means by that.

On Friday, Mark Esper, the US defence secretary, said he “interprets” Mr Trump’s remarks to mean the military should deny Isis access to the oil fields.

US-backed Syrian Kurdish forces currently control the oil, supported by a small contingent of US troops.

A quiet arrangement has existed between the Kurds and the Syrian government, which sees Damascus buying the surplus through middlemen. 

It comes after Syria’s president, Bashar al-Assad, praised Mr Trump as the “best” US president he has dealt with, “because he is the most transparent”.

“He’s the best American president,” Mr Assad told NBC News from Damascus. ”Not because his policies are good, but because he’s the most transparent president.”

Additional reporting by Associated Press