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

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

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

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 in recent weeks 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 defense of the capital. The larger contingent is Iraqi," the source said on condition of anonymity.

Iran is a key ally of the Syrian government, and it 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-Qaida affiliate Al-Nusra Front.

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 further northeast.

The Al-Watan newspaper, which is close to the government, said IS had begun a "violent attack on Hasakeh" but criticized 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 neighboring Iraq, IS fighters last month used an unprecedented wave of suicide truck bomb attacks to seize Ramadi, the capital of the country's largest province Anbar in a major setback to 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 U.S.-led coalition leveled one of the jihadists' largest car bomb factories in Iraq, which was packed with tanks, Humvees, and explosives, officials said.

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

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.

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