The jihadis have now advanced to “500 meters away from the entrance of Hassakeh, 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 ISIS 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 Wednesday.
Regime helicopters meanwhile dropped barrel bombs on jihadi positions, he said.
Control of Hassakeh city is split between regime and Kurdish forces, who have fought ISIS elsewhere in the province.
If ISIS seizes Hassakeh it would be the second Syrian provincial capital to fall under its control, after Raqqa, the group’s bastion to the west.
It would also be the third provincial capital lost by the regime. The northwestern city of Idlib was seized by rebels in March.
Fighting Wednesday around Hassakeh left 27 regime fighters and 26 jihadis dead, the Observatory said.
All electricity and methods of communications in Hassakeh 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 Hassakeh city following ISIS mortar fire, he told AFP from the city of Qamishli.
The Al-Watan newspaper, which is close to the government, said ISIS had begun a “violent attack on Hassakeh” but criticized Kurdish forces for failing to support regime troops.
Local activists in Hassakeh have said the Kurdish YPG militia has been engaged in a concerted drive to round up young men for military service, as the assault continues.
ISIS began its assault on Hassakeh on May 30 and launched a simultaneous offensive against rebels in Aleppo province.
However, the YPG militia has also been on the offensive against ISIS – its fighters are closing in on the border town of Tal Abyad, in Raqqa province.
A Turkish government official said more than 3,300 Syrians have crossed into Turkey in the past two days fleeing the fighting.
The refugees entered Turkey at the Akcakale border crossing, the official said Thursday on condition of anonymity. He also cited a U.S.-led coalition airstrike in rural areas near Tal Abyad.
Tal Abyad is a key link between Turkey and the city of Raqqa.
Also, Syria’s defense minister has visited army units to the east of Homs, state television reported Thursday, in what appeared to be the latest in a series of morale-boosting trips by senior officials to military outposts.
Gen. Fahd Freij, who is armed forces deputy commander as well as defense minister, told the troops in the eastern Homs countryside he was confident in their ability to defend Syria from what he called “terrorism and its supporters.”
Homs lies 150 km to the west of Palmyra, which was seized last month by ISIS jihadis.
Homs governor Talal al-Barazi voiced confidence in the military, saying he expected it to retake Palmyra, also known as Tadmur, at “the appropriate time” and that it would win back land all the way to the town of Sukhna to the east.
“I believe that all Syrians in areas near to the front lines with the gunmen are concerned, and the concern grows as [gunmen] get near a place, because this criminal monster causes worry for any civilian in any place,” he told Reuters Television in an interview in Homs.
But he added: “There is no collapse, no psychological defeat.” He said the army had decided against fighting ISIS in Palmyra itself “to avoid civilian casualties and to protect the ancient city.”
Elsewhere, at least 14 civilians, including seven children, were killed when regime helicopters dropped barrel bombs on northern Aleppo province, the Observatory said.
In Turkey, Syria’s opposition National Coalition met with U.N. peace envoy Staffan de Mistura as part of “ongoing discussions about a political solution,” a coalition spokesman said.