BEIRUT  - Militants of the Islamic State (IS) group seized a string of villages in northern Syria on Wednesday in fighting that left 52 people dead, a monitoring group said.

The militants captured eight villages between second city Aleppo and the Turkish border, buoyed by their successes in neighbouring Iraq, the Syrian Observatory for Human Rights said.

They killed at least 40 fighters of rival rebel groups and captured at least 50 more for the loss of 12 of their own men, the Britain-based group said.

IS already controls almost all of the Euphrates valley provinces of Raqa and Deir Ezzor and has been making gains further west at the expense of rival rebel groups which it has been fighting since December.

The rival rebels, many of them from more moderate Islamist groups, have been weakened by the defection late last month of Al-Qaeda affiliate Al-Nusra Front, which had made common cause in the battle against IS.

The Observatory said the IS capture of the villages was a strategic prize, because it would open the way for the group to attack the towns of Marea and Azaz.

Marea is a stronghold of the Islamic Front, a coalition of Islamist groups that is among those fighting against IS.

Azaz sits next to the border crossing with Turkey, which would be a valuable asset to IS as it seeks to expand its self-declared "caliphate" in the territory it holds in Syria and neighbouring Iraq.

The Islamic State emerged from Al-Qaeda's one-time branch in Iraq, and initially fought alongside Syria's opposition, including more moderate rebels and Al-Nusra fighters.

But its abuses against civilians and rival fighters, as well as its bid to dominate control of captured territory, sparked a backlash.

The group was pushed out of much of the territory it held in Aleppo province by the coalition of moderate and Islamist rebels that turned against it in January.

But it has been able to recapture some of that lost ground assisted by the decision of the Islamist Dawud Brigade to pledge allegiance to it.