Clashes in Syria's northeast between regime troops and forces aligned with the Kurdish-led Syrian Democratic Forces killed two from each side on Tuesday, a war monitor said.
Syria's Kurds set up a semi-autonomous administration in the country's northeast in 2013 after government troops withdrew. The SDF, a key US partner in fighting the Islamic State jihadist group, is the Kurdish administration's de-facto army.
Clashes between Kurdish and regime forces are rare in the region.
"Two regime soldiers were killed and others were wounded" while two members of an SDF-affiliated "military council" in Tal Tamr died after an "armed clash" in the area, the Syrian Observatory for Human Rights said.
The official SANA news agency said that a "patrol of US forces accompanied by members of the SDF militia tried to penetrate points controlled by the Syrian army" in Hasakeh province.
It did not mention whether there were victims but said the SDF attacked after soldiers blocked the patrol's passage.
US troops are in Syria as part of an anti-jihadist coalition.
The SDF confirmed the toll in a statement. It did not mention the presence of Americans, and called the incident "a dangerous provocation by the Syrian regime".
The war in Syria is estimated to have killed nearly half a million people and displaced millions more since it began with a brutal crackdown on anti-government protests in 2011.
It quickly spiralled into a complex conflict that pulled in numerous actors, including jihadist groups and foreign powers.
Russia intervened militarily in Syria more than six years ago to shore up President Bashar al-Assad.
Neighbouring Turkey views some Syrian Kurdish fighters as "terrorists" and has launched several operations against them.