New fighting broke out overnight between Kurdish and regime forces in the flashpoint northern Syrian city of Hasakeh, a monitor said on Monday, as Russian officials pursued mediation efforts.
The fresh violence came amid conflicting reports on whether a truce had been agreed on Sunday; with Kurdish sources denying reports by Syrian military sources and state media that a ceasefire was reached.
After hours of calm late Sunday, "clashes broke out after midnight in the southern district of Ghweiran and Al-Masaken" in central Hasakeh, the Syrian Observatory for Human Rights said.
"Regime forces still control most of Ghweiran," Observatory head Rami Abdel Rahman told AFP, adding that the Russian peace efforts were still under way.
A local journalist working with AFP confirmed that clashes were heard in southern and central Hasakeh and that pro-government fighters could be seen retreating from parts of Ghweiran.
Hasakeh city, capital of the northeastern province of the same name, is already mostly controlled by Kurdish forces although the majority of its residents are Arab.
Syria's regime and Kurdish forces share a common enemy in the Islamic State (IS) militant group, but there have been growing tensions between the two groups in Hasakeh.
Clashes erupted last week between the Kurdish police known as the Asayesh and the pro-government National Defence Forces militia (NDF), and escalated Thursday when regime warplanes bombarded Kurdish-held positions in the city for the first time.
That action came close to sparking the intervention of US warplanes when Washington warned against strikes that might endanger its military advisers with the Kurds on the ground.
The fighting in Hasakeh has also drawn in the Syrian army and the powerful Kurdish People's Protection Units (YPG).
Kurds, who run autonomous institutions across much of northeast Syria, have often called for the dissolution of the NDF.
In an attempt to end the violence, Russian officials based at Hmeimim military airport on the coast flew to the nearby city of Qamishli to hold mediation talks.
A Syrian military source told AFP late Sunday that a ceasefire deal had been reached to end hostilities, hand back any new positions seized during the clashes and evacuate the wounded to Qamishli.
On Monday, a Syrian military source accused the YPG of "violating the agreement by refusing to allow safe passage for the dead and wounded to reach Qamishli".
But Kurdish sources have insisted that no agreement was reached at all.
One official in the Kurdish autonomous administration said talks "via indirect mediation" were still discussing "the dissolution of the NDF in Hasakeh".
He said Kurdish forces "have no plans as of now to seize control of all of Hasakeh"