At least 150 US soldiers have entered Kurdish-controlled territory in northeastern Syria, the government's official news agency has reported.
The state news outlet SANA on Thursday said officials at the country's foreign ministry rejected the presence of the soldiers at the Rmelan area calling it "illegitimate".
"This intervention is rejected and illegitimate, and it happened without the Syrian government’s approval," the source said, according to SANA.
The government said the move by the US was a "blatant act of aggression that constitutes a dangerous intervention and a gross violation of the Syrian sovereignty", further calling for international action to prevent it.
The US has previously denied operating a base in Rmelan despite satellite imagery showing the expansion of an airstrip in the area.
US-backed Kurdish People's Protection Units (YPG) had previously said the the US had taken control of the disused airfield, just kilometres away from the Turkish and Iraqi borders.
Media activists said at the time said the YPG and its allies had handed over the base to the US so that it could provide air cover to Kurdish forces in their war against the Islamic State of Iraq and the Levant (ISIL) group.
US troops have been stationed in neighbouring Iraq since 2014 where they are training Iraqi soldiers and Peshmerga to battle ISIL (also known as ISIS).
Special forces soldiers from the US have also carried out a number of raids against ISIL targets in Syria and Iraq.
Source: Al Jazeera