Beirut (AFP) - The Islamic State jihadist group launched a major offensive Wednesday to try to capture a strategic town on the Syrian-Turkish border, leaving dozens dead in clashes, a monitor said.

"Fighters from the Islamic State group started a huge assault towards Ras al-Ain and were able to take over a village nearby," the Syrian Observatory for Human Rights said.

The offensive is a preemptive strike against Kurdish militia who were planning an attack on the IS-held town of Tal Abyad farther west along the border, Observatory head Rami Abdel Rahman told AFP.

Tal Abyad is an Arab and Kurdish town in the Syrian province of Raqa used by IS jihadists as a gateway from Turkey.

At least 12 fighters from the Kurdish People's Protection Units (YPG), which control Ras al-Ain and the surrounding villages, were killed in the IS onslaught, according to Abdel Rahman.

"This is a big hit to the morale of Kurdish fighters," he said.

He was unable to give an exact death toll for the jihadists but said that including IS casualties scores had been killed.

A spokesman for the Democratic Union Party (PYD), the YPG's political arm, confirmed an intense battle was raging around the town.

Ras al-Ain, in Hasakeh province, was the scene of major fighting in 2013 before Kurdish forces ousted rebels and Al-Qaeda-linked jihadists from the town, which has a border crossing with Ceylanpinar in Turkey.

Kurdish fighters are also locked in clashes with IS around the strategic town of Tal Tamr, just southeast of Ras al-Ain, which lies near a key road that links to their Iraqi bastion of Mosul to the east.

The IS offensives come just weeks after Kurdish militia backed by Iraqi peshmerga fighters and Syrian rebels drove the extremists out of Kobane farther west along the Turkish border.

The town, which was devastated by months of fighting and US-led coalition air strikes, became a prominent symbol of resistance against the jihadists.

Kurdish and allied forces have since taken much of the surrounding countryside in northern Aleppo province and have begun pushing east into neighbouring Raqa province, home to IS's self-proclaimed "capital."

IS has seized large parts of Syria and neighbouring Iraq and imposed a harsh interpretation of Islamic law.

Foreign jihadists have flocked to Syria, often crossing over from Turkey, since the country's conflict began in March 2011 as a popular revolt which later escalated into a full-blown civil war.