Islamic State group jihadists, emboldened by a string of battlefield victories, advanced Thursday to the gates of the Syrian city of Hasakeh after intense fighting with regime troops.

In neighbouring Iraq, security forces foiled car bomb attacks by IS on two military bases west of Baghdad, a day after US-led coalition warplanes destroyed a massive jihadist bomb factory.

Despite nine months of US-led air strikes, the militants have made new territorial gains in recent weeks, seizing areas including the ancient Syrian city of Palmyra and Ramadi, capital of Iraq's vast Anbar province.

Now the jihadists have advanced to "500 metres (550 yards) away from the entrance of Hasakeh, after fierce clashes against regime forces south of the city," said Rami Abdel Rahman, director of Britain-based monitoring group the Syrian Observatory for Human Rights.

He said IS had seized all military posts in that area of northeastern Syria, including an unfinished prison building and a power plant, after at least six suicide bombers struck on Wednesday.

Regime helicopters meanwhile dropped barrel bombs on jihadist positions, he said.

Control of Hasakeh city is split between regime and Kurdish forces, who have fought IS elsewhere in the province.

If IS seizes Hasakeh it would be the second Syrian provincial capital to fall under its control, after Raqa, the group's bastion in northern Syria.

It would also be the third provincial capital lost by the regime. The northwestern city of Idlib was seized by rebels in March.

Iran and Iraq have deployed thousands of fighters in Syria to help the regime defend Damascus and its surroundings, a security source told AFP on Wednesday.

"Around 7,000 Iranian and Iraqi fighters have arrived in Syria over the past few weeks and their first priority is the defence of the capital. The larger contingent is Iraqi," the source said on condition of anonymity.

Iran, a key ally of the Syrian government, has provided Damascus with financial and military support.

But in recent months, the Syrian government has lost territory in several parts of the country to both IS and rebel groups including Al-Qaeda affiliate Al-Nusra Front.

- 'Violent attack' -

Fighting on Wednesday around Hasakeh left 27 regime fighters and 26 jihadists dead, according to the Observatory, which relies on a network of sources on the ground.

All electricity and methods of communications in Hasakeh city have been cut off, said activist Arin Shekhmos, citing residents who fled the provincial capital.

Other residents have moved to the northern and western Kurdish-controlled parts of Hasakeh city following IS mortar fire, he told AFP from Qamishli farther northeast.

The Al-Watan newspaper, which is close to the government, said IS had begun a "violent attack on Hasakeh" but criticised Kurdish forces for failing to support regime troops.

IS began its assault on Hasakeh on May 30 and launched a simultaneous offensive against rebels in northern province of Aleppo.

The attacks follow the jihadists' capture last month of Palmyra and its 2,000-year-old UNESCO World Heritage site in central Syria. It has also expanded its control along the Syrian-Iraqi frontier.

In neighbouring Iraq, IS last month used an unprecedented wave of suicide truck bomb attacks to seize Ramadi, capital of the country's largest province, in a major setback for the government.

On Thursday, the Iraqi army said it had used anti-tank missiles to repel suicide bombers driving explosives-rigged vehicles towards two military bases west of Baghdad.

The foiled attacks came a day after an air raid by the US-led coalition levelled one of the jihadists' largest car bomb factories in Iraq, which was packed with tanks, Humvees, and explosives, officials said.

US Deputy Security of State Antony Blinken said Wednesday that 10,000 IS members had been killed since the start of the US-led air campaign in Iraq and Syria.

The coalition said it had conducted 17 air strikes against IS positions in Iraq and Syria over a 24-hour period through Thursday morning.

It said the targets included tactical units near Hasakeh, Raqa and Ramadi, as well as vehicles, weapons and excavators used by the jihadists.

More than 220,000 people have been killed in Syria since anti-government protests broke out in March 2011 and escalated into a full-blown civil war that brought jihadists streaming into the country.

At least 14 civilians, including seven children, were killed Thursday when Syrian regime helicopters dropped barrel bombs on northern Aleppo province, the Observatory said.

In Turkey, Syria's opposition National Coalition met with UN peace envoy Staffan de Mistura as part of "ongoing discussions about a political solution", a coalition spokesman said.