Hundreds of ISIS fighters have launched an attack on Kurdish forces in northeastern Syria, triggering fierce fighting that has killed dozens on both sides, a group monitoring the war said on Wednesday.

The Kurdish YPG militia, backed by U.S.-led air strikes, had made significant gains in recent weeks against the self-proclaimed Islamic State of Iraq and al-Sham (ISIS) in the region, cutting an important supply route from territory controlled by the militant group in Iraq.

ISIS appeared to try to seize back the initiative on Tuesday, attacking Kurdish forces using tanks and heavy weapons around Ras al-Ayn town near the Turkish border, the Britain-based Syrian Observatory for Human Rights said.