Islamic State militants fighting in Syria on Friday reached the southern gates of the predominantly Kurdish northeastern city of Hassakeh amid intense air raids and shelling, Syrian activists and the extremist group's radio station said.

The Islamic State group has been attacking Hassakeh city since May 30, facing stiff resistance from government forces. The city has been split between government forces and Kurdish fighters who have been attacking IS positions elsewhere in the province.

The Britain-based Syrian Observatory for Human rights said that IS fighters captured a power station and an unfinished juvenile prison building just south of Hassakeh.

IS radio station Al-Bayan said the militants were about one kilometer (mile) from the southern entrance in the city, claiming they have killed dozens of soldiers.

The Observatory and the Local Coordination Committees say government warplanes repeatedly attacked IS forces near Hassakeh on Friday. The Observatory said 71 government troops as well as 48 IS fighters were killed in Hassakeh over the past week.

Members of the Kurdish militia known as the People's Protection Units, or YPG, have not joined the battle in the city so far, but one of its commanders, Polan Jan, said in comments posted on the group's official Facebook page that "we will not allow Daesh or others to control our Kurdish city of Hassakeh." He used an Arabic acronym to refer to the group.

YPG have been advancing in northern Syria against IS fighters for weeks, under the cover of the U.S.-led coalition that is bombing IS positions.

Elsewhere, the Kurdish fighters are closing in on the Islamic state group in Tel Abyad, capturing towns and villages in the oil-rich swath in the country's northeast — again, supported by U.S.-led airstrikes.

Tel Abyad is a key link between Turkey and the northern Syrian city of Raqqa, the IS's de-facto capital. The Raqqa-based media collective called "Raqqa is Being Silently Slaughtered" said that U.S.-led coalition warplanes attacked Tel Abyad on Friday.

On Thursday, a Turkish government official said that more than 3,300 Syrians have crossed into Turkey in the past two days fleeing fighting between IS and the YPG in the Tel Abyad region.