Fighting between Kurdish and pro-government factions in the northeastern Syrian town of Qamishli intensified on Thursday, in what is likely the bloodiest such violence of the conflict there.

Kurdish police, known as Asayish began clashing with forces loyal to Syrian president Bashar al-Assad on Wednesday morning, reportedly after a dispute at a checkpoint manned by the Assadist National Defense Forces (NDF) militia.

The Asayish were quickly reinforced by their SWAT-style squad, known as HAT, and Kurdish People's Protection Units (YPG) fighters. The two sides exchanged machine gun and rocket fire, with some of the fiercest fighting focused on government-held Allya Prison, which was eventually overrun by Kurdish forces after more than 40 NDF members surrendered.

A video released by the YPG was described as showing the incident, including the badly battle-damaged prison. Both sides have suffered multiple casualties, although differing figures have been circulated. At least three Asayish members died, while Kurdish forces killed four NDF members and injured a number of others, the UK-based Syrian Observatory for Human Rights said on Thursday morning.

Pro-government snipers also shot dead two civilians, according to the pro-Kurdish ARA News.

Syrian troops went on to launch mortar shells at the town from a base on the outskirts, causing a number of civilian casualties, local news outlets reported.

Asayish videos showed masked HAT members in action, firing RPGs, assault rifles, and truck-mounted heavy machine guns as well as operating a small reconnaissance drone. Another showed the Asayish flag being raised above the prison complex.

An unverified picture posted by a YPG member and Kurdish activists on social media showed smiling Kurdish fighters destroying a large picture of Assad.

The two sides reportedly agreed on a truce on Wednesday night, but hostilities quickly resumed.

Control of Qamishli has been split between Syrian government and Kurdish administration since 2012. An uneasy peace has largely been maintained, with Kurdish and government forces even allying against advances from the Islamic State (IS) group, but tensions have spilt over into violence on a number of occasions.

The YPG make up the largest segment of the Syrian Democratic Forces (SDF), an umbrella group which receives significant American support in its fight against IS. But this growing strength and increasingly cozy relationship with the anti-Assad US may also have alarmed government forces.