BEIRUT (REUTERS) - A Syrian Kurdish militia has captured a town from the Islamic State in Iraq and Syria (ISIS) militant group in the northeast, a group monitoring the war reported on Wednesday, compounding recent losses for the militants in a strategic corner of Syria.

The capture of the town of Al-Mabroukah by the YPG militia opens the road for its forces to advance towards the Islamic State's stronghold in Raqqa province, the Syrian Observatory for Human Rights reported.

While losing ground in the northeast, ISIS has launched a new offensive in central Syria, where it has seized wide areas from the Damascus government including the ancient city of Palmyra, also known as Tadmur.

The YPG, supported by United States-led air strikes, have been making steady gains this month against ISIS in the northeast, which is important in the battle against the jihadists due to its location bordering territory held by the group in Iraq.

The Syrian military has also captured territory from ISIS in the same region, the Observatory says.

The Observatory said the YPG had seized control of 4,000 square kilometres in this month's advance.

The YPG has emerged as the only notable partner on the ground for the US-led alliance bombing of ISIS in Syria. The US has rejected the idea of partnering with President Bashar al-Assad, saying he is part of the problem.

The capture of Al-Mabroukah could now allow the YPG to advance towards the ISIS-held town of Tel Abyad at the border with Turkey. "The goal is Tel Abyad," said Rami Abdulrahman, who runs the Observatory.