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More than a dozen Syrian rebel groups accused the country's main Kurdish militia, People's Protection Units (YPG), of committing "ethnic cleansing" against Arabs and Turkmen in northern Syria, a charge strongly denied by the Kurds.

The accusation came as Kurdish fighters reached the outskirts of a strategic Islamic State-held town on the Turkish border.

Thousands of refugees, most of them Sunni Arabs, have fled to Turkey in the past two weeks as the YPG has pushed to dislodge Islamic state (IS) militants from Tal Abyad, aiming to sever a key supply line for the extremists' nearby de facto capital, Raqqa.

Tal Abyad, which lies on the Turkish border, is a mixed town with a population that includes Arabs, Turkmen and Kurds.

The Kurdish advance, coming under the cover of intense US-led coalition airstrikes in the area, has aggravated sectarian and ethnic tensions in the region.

"YPG forces ... have implemented a new sectarian and ethnic cleansing campaign against Sunni Arabs and Turkmen under the cover of coalition airstrikes which have contributed bombardment, terrorising civilians and forcing them to flee their villages," the statement issued by rebel and militant groups said.

The 15 groups, including the powerful ultraconservative Ahrar al-Sham and Jaish al-Islam, said the alleged "cleansing" is concentrated in the northeastern predominantly Kurdish province of Hasakah and in Tal Abyad.

They said it was caused by the Kurdish advance and US-led airstrikes.

It charged that the "cleansing" was part of a plan by the Kurdish Democratic Party (PYD) to partition Syria. The YPG, or People's Protection Units, is the armed wing of the PYD.

The movement is affiliated with the Kurdish PKK, which has waged a long and bloody insurgency in southeastern Turkey, and is designated as a terrorist organisation in Turkey and several other Western countries.

Losing Tal Abyad, some 80 kilometers (50 miles) north of the IS stronghold of Raqqa, would deprive the extremist militant group of a direct route to bring in new foreign militants or supplies.

The Kurdish advance, coming under the cover of intense US-led coalition airstrikes in the area, would also link their two fronts and put even more pressure on Raqqa.

Around 16,000 people have fled the fighting in the past two weeks. On Monday, up to 3,000 more refugees arrived at the Akcakale border crossing. Thick smoke billowed across the border as US-led coalition aircraft targeted IS militants in Tal Abyad.

IS has massacred hundreds of Shia and other opponents and boasted of the killings in online videos.

The extremist group has also kidnapped hundreds of members of the Yazidi religious minority - who are ethnic Kurds - and enslaved women and girls.

Hundreds of thousands of Iraqis, including Sunni Arabs, have fled the IS advance over the past year and taken refuge in the largely autonomous Kurdish region of northern Iraq.