Syria on Sunday condemned an agreement between Kurdish-led forces in the country's northeast and a US oil company, describing it as "theft" and an "affront to national sovereignty".
The foreign ministry denounced "an agreement signed by the SDF militia and a US oil company to steal Syrian oil... supported by the US administration", in a statement quoted by the official SANA news agency.
The SDF is the Syrian Democratic Forces, a Kurdish-led paramilitary alliance that backs a semi-autonomous administration in northeastern Syria and controls the country's biggest oilfields.
Senior US officials have confirmed an agreement to "modernise" the fields, without naming the US company or providing other details.
The Syrian foreign ministry's statement decried "an agreement between... thieves who steal and thieves who buy".
It also decried "the hostile US position towards Syria, the theft of the Syrian people's riches and its hindrance of the state's reconstruction efforts".
Senator Lindsey Graham, a longtime supporter of the Syrian Kurds, told a congressional hearing Thursday that he had spoken about the deal with SDF commander General Mazloum Abdi.
"Apparently they've signed a deal with an American oil company to modernise the oil fields in northeastern Syria," Graham said.
Asked by Graham if the US was supportive of the deal, Secretary of State Mike Pompeo said: "We are."
"The deal took a little longer, senator, than we had hoped and we're now in implementation. It can be very powerful," Pompeo said.
A US-led military coalition strongly backed the SDF against the Islamic State group in Syria, helping the Kurdish-led outfit seize the jihadists' final patch of territory there in March last year.
Syria's war began in 2011 with the violent suppression of peaceful protests and snowballed into a multi-fronted conflict pulling in multiple external powers.
It has resulted in the country losing tens of billions of dollars in oil revenues.
The fighting has often destroyed hydrocarbon infrastructure, which has been coveted by the various belligerents.
Before the civil war, Syria produced nearly 400,000 barrels of oil per day, but output has collapsed during the conflict.