A fighter of the Islamic State (ISIS)
A fighter of the Islamic State (ISIS)Reuters

The “Islamic State” (IS or ISIS) group on Tuesday freed more than 70 Kurdish school children its fighters kidnapped in northern Syria in May, the Syrian Observatory for Human Rights said, according to AFP.

The Britain-based group said parents had reported the release of the children, who are believed to be aged between 13 and 15.

The group of 70 were among 153 school children, most of them boys, abducted by ISIS jihadists on May 29 in the northern province of Aleppo.

They were intercepted and taken hostage en route to their hometown of Ain al-Arab after sitting school exams in Aleppo.

They were kidnapped around the area of Minbej, which has been targeted in recent days in airstrikes by a U.S.-led coalition fighting jihadists, AFP reported.

In the weeks after the children were abducted, five students managed to escape and another 37 were released in batches, including the 10 girls among the group, the Observatory said.

There were no immediate details on why the group decided to free the 70 additional hostages on Tuesday, and the Observatory said around 30 more children were still being held.

Ain al-Arab, known to the Kurds as Kobane, is now a major battleground between Kurdish fighters and ISIS.

Located strategically on the Syria-Turkey border, it is the country's third biggest Kurdish town.

ISIS fighters have been advancing towards it for the past two weeks, and are now just three kilometers (less than two miles) from the town in places, according to the Observatory.

Kurdish leaders have warned of a renewed threat of genocide should IS gain control of Kobane.

U.S. and allied forces from Arab states have been bombing IS positions in Syria since last week, after the U.S. expanded its air campaign against the terrorist group; until then the operation had been focused on blunting the Islamists' advances in only Iraq.

While airstrikes in Iraq - coupled with a ground coalition made up of Kurdish Peshmerga forces, Iranian-backed Shia militias and the Iraqi army - have succeeded in slowing down and even halting IS's advances there, airstrikes in Syria appear to have been ineffective thus far.