In this June 15, 2015 file photo taken from the Turkish side of the border between Turkey and Syria, in Akcakale, southeastern Turkey, Kurdish fighters with the YPG, wave their yellow flag in the outskirts of Tal Abyad, Syria.

A bomb placed on a motorcycle exploded on Sunday in a Kurdish-held Syrian town along the Turkish border, killing at least three and wounded 20 others, a monitor said.

The UK-based Syrian Observatory for Human Rights said a huge blast was heard when the bike blew up in the center of Tel Abyad, a town captured by Kurdish-led forces from Islamic State of Iraq and Syria (ISIS) militants in June with help from U.S.-led air strikes.

The target was a checkpoint of Kurdish YPG forces, another source said.

Although Kurdish forces have reversed the momentum of the ultra-hardline Islamist militants along the Turkish border, and pushed them from areas in the northern provinces, ISIS fighters continue to carry out lighting attacks, such as ambushes and suicide car bombings.

Last week ISIS attacked Kurdish forces and a coalition of Arab tribes in Ain Issa, a town further south in Raqqa province also seized by the Kurdish-led forces last summer and within 50 km of Raqqa city, ISIS’ de facto capital.

The YPG fighters, who have emerged as the most credible ally of the U.S.-led campaign on the ground in Syria, recently joined forces with several Arab tribes in a new U.S.-backed coalition called the Democratic Forces of Syria.

They seized over a week ago several ISIS-held villages in Hasaka province, the most important of which was al Houl, in a push further south into ISIS-controlled territory.

Separately, the Syrian army which has also been fighting ISIS across northern Syria said on Sunday it had taken control of a series of heights known as the Hayel in the southern part of the ancient city of Palmyra in central Syria.

The Syrian army, backed by some of the heaviest Russian attacks on the hardline Islamist group, has been waging a major offensive to recapture the historic city that was seized last May.

The army also said on Sunday it was closing in on the town of Maheen in the southwest of Homs which was captured by militants from government forces early this month.

These areas have seen intensified Russian air bombardments in recent days, say rebels. The Russian military has also said they have intensified raids against militant hideouts across Syria.

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