Throughout its spectacular rise to power during 2013 and its announcement of an Islamic "caliphate" straddling Iraq and Syria in the summer of 2014, IS has presented itself as unstoppable.

The ultra-radical Sunni group has used terror and mass executions to seize territory in Syria's north, including the group's de facto capital in Raqa, and east, where it has captured most of resource-rich Deir Ezzor province.

But in recent months, IS has experienced a series of defeats at the hands of Kurdish militia in north and northeast Syria. The jihadists' only major victory was its capture of the ancient town of Palmyra in May.

Now the IS assault on Hasakeh city, which began on June 25, has ended in defeat after 33 days of fighting.

"The Syrian army pushed IS out of Zuhur, the last neighbourhood where it was present in Hasakeh city, on Tuesday" said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.

At least 287 IS fighters were killed in the Hasakeh assault, either in clashes with regime troops and the Kurdish People's Protection Units (YPG) or in air strikes by a US-led coalition, the Observatory said.

- 'Child soldiers' -

IS's death toll also included at least 26 child soldiers, according to the monitor, which uses a network of on-the-ground sources throughout war-torn Syria.

"IS is depending more on children for suicide operations. And we saw the effect of that, since 10 percent of IS members killed in the fight for Hasakeh city were children," Abdel Rahman said.

The jihadist group had used at least 21 car bombs and "several suicide bombers" in its advance into the city.

According to the Observatory, 120 regime loyalists and dozens of YPG fighters were also killed in the battle for Hasakeh.

State news agency SANA said Syria's armed forces "dealt great blows to the Daesh (IS) terrorists... in a special operation against the terrorist hotbeds" in Zuhur on Tuesday.

Control of Hasakeh city is divided between forces loyal to Syrian President Bashar al-Assad and the Kurdish YPG.

IS had attacked regime-controlled neighbourhoods in the city's south, but the YPG helped push IS out of several of these areas.

- String of IS losses -

Since the beginning of Syria's uprising in March 2011, the country's Kurdish minority had adopted a neutral position, siding neither with the regime nor with the swelling rebellion.

When government forces withdrew from Kurdish areas in some parts of the north and northeast, Kurdish authorities established autonomous local administrations to fill the void.

As IS sought to advance in northern Syria, the YPG became one of the main fighting forces pushing back against the jihadists. They expelled IS from Kobane, a key town on the Turkish border, in late January.

Since then, Abdel Rahman told AFP, "we haven't seen any major IS victories with the exception of the battle at Palmyra. There has been a continuous retreat for IS forces since February."

He even described the group's month-long assault on Hasakeh as a "major loss", as the group lost many military commanders in the fight, including its provincial "wali" or governor.

On Monday, Kurdish forces also cut off an IS supply route between the jihadists' bastion of Raqa and their military fronts in Aleppo province. And in June, Kurdish militia dealt the jihadists a resounding blow at Tal Abyad, a border town that IS had used as a transit point for supplies and fighters.

According to Abdel Rahman, the loss of Tal Abyad has seriously damaged IS's access to both fighters and supplies, and has led to the group's recent losses.

Air strikes by the international coalition fighting IS in Syria and Iraq have also contained the group, he said.

More than 230,000 people have been killed in Syria's multi-front war.