Soldiers from the Kurdish YPG have launched a major assault to seize the last government-controlled areas of the northeastern Syrian city of Hasaka, after a Russian mediation team failed to mend the rift between the two sides.

The YPG began the offensive after midnight to take the southeastern district of Nashwa, close to where a security compound is located near the governor's office close to the heart of the city, according to YPG sources and residents.

The YPG, or the People's Protection Units, has ties to Turkey's outlawed Kurdistan Workers Party (PKK).

It had earlier captured Ghwairan, the only major Arab neighbourhood in Hasaka still in government hands.

Monday's offensive comes just hours after Syrian state media said a truce had been reached between Kurdish and government forces to evacuate the wounded from Hasaka.

Kurdish sources said on Sunday that no deal had been finalised.

Violent confrontation

The fighting in Hasaka, which is divided into zones of Kurdish and Syrian government control, marks the most violent confrontation between the Kurdish YPG and the Syrian army in more than five years of civil war.

Goverment fighter jets last week bombed the YPG for the first time during the war, prompting a US-led coalition to scramble aircraft to protect American special operations forces on the ground in the area.

"This is quite a significant development because the Kurds now seem to be determined to drive government troops out of the city and control it," said Al Jazeera's Hashem Ahelbarra, reporting from Gaziantep, near the Turkey-Syria border.

"The Kurds are saying that they're not going to sign any agreement in the future with [President] Bashar al-Assad, and that it's just going to be a matter of hours or days before Hasaka is under their control."

The YPG is at the heart of a US-led campaign against the Islamic State of Iraq and the Levant (ISIL) group in Syria.

It controls swaths of territory along the northeastern border with Turkey - from Hasaka to Afrin - which its political wing has claimed as an autonomous region now called Rojava.

Our correspondent said: "[Kurdish] fighters, as we speak, are surrounding government buildings in Hasaka, giving soldiers an ultimatum: Either you surrender, or you will be killed."

Syrian state media accused the YPG-affiliated security force known as the Asayish of violating a ceasefire and said its members had torched government buildings in Hasaka.

It accused the Asayish of igniting the violence through escalating "provocations", including the bombing of army positions in Hasaka, and said the Asayish aimed to take control of the city.

'We will not retreat'

Late on Sunday night, Kurdish forces distributed leaflets across Hasaka and used mosque loudspeakers to call on army personnel and pro-government militias to hand over their weapons or face death.

"To all the elements of the regime and its militias who are besieged in the city you are targeted by our units," leaflets distributed by the YPG said.

"This battle is decided and we will not retreat ... We call on you to give up your weapons or count yourselves dead."

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The YPG appeared intent on leaving a nominal Syrian government presence confined to within a security zone in the heart of the city, where several key government buildings are located, Kurdish sources said.

Clashes in Hasaka erupted last week after Kurdish forces demanded the dismantlement of a pro-government group, the National Defence Forces, in the area.

Russian mediation

In an attempt to calm tensions, a delegation of Russian officials arrived in Qamishli on Saturday from the coastal Hmeimim military airport for talks with the two sides.

After two days of negotiations mediated by Russia, a Syrian military source told AFP that a deal was struck between government forces and Kurdish fighters - a claim Kurdish sources denied.

The three-point agreement calls for a "halt to all hostilities and the return to regime forces of any positions seized by Kurdish fighters" since Wednesday, according to the military source.

The deal also stipulated that casualties would be transferred north to Qamishli.

A Kurdish military source in Hasaka told AFP that an agreement had not been established on any of the three points.

The pro-government Al Masdar news website reported on Sunday that Russian mediation efforts had failed to halt fighting after the Syrian government refused Kurdish demands that it withdraw its forces from Hasaka and Qamishli.

Additional negotiations were set to take place on Monday.

Thousands of civilians in the ethnically mixed city have fled to villages in the countryside as the fighting intensified, residents said.

Tacit understandings

The confrontation appears to have undone tacit understandings between the YPG and the Syrian army that had kept Hasaka relatively calm.

Critics and residents say the YPG was handed weapons and territory by the Syrian army at the start of the conflict, as Assad sought to focus on crushing the mainly Sunni Arab rebels who sought to topple him.

After the clashes began last week, Hasaka's governor told state media the military had armed the YPG with weapons and tanks to fight "jihadist elements" and had not expected them to turn against the state.

Hasaka's population has been swelled by displaced Syrians fleeing areas that fell under the control of ISIL, also known as ISIS.

It is broadly divided along ethnic lines, with Kurds mainly in the city's eastern neighbourhoods and Arabs in the southern parts.