A ministry statement, published on state media on Sunday, did not name the firm involved in the deal with the US-backed militants, Reuters reported.
“This agreement is null and void and has no legal basis,” it said.
A US senator and Secretary of State Mike Pompeo had referred to an oilfields deal between the SDF and a US firm during a US Senate Foreign Relations Committee hearing on Thursday.
Republican Senator Lindsey Graham said during the committee hearing that SDF head Mazloum Abdi informed him that a deal had been signed with an American company to “modernize the oil fields in northeastern Syria”, and asked Pompeo whether the administration was supportive of it.
“We are,” Pompeo responded during the hearing streamed live by PBS. “The deal took a little longer ... than we had hoped, and now we’re in implementation.”
US President Donald Trump has said that despite a military pullback from northeast Syria, a small number of American forces would remain “where they have oil”. The Pentagon said late last year that oilfield revenues would go to the SDF.