Kurdish People's Protection Units fighter walks near residents who had fled Tel Abyad, as they re-enter Syria from Turkey after the YPG took control of the area, at Tel Abyad town, Raqqa governorate, Syria Thomson Reuters ANKARA (Reuters) - Turkey has warned the United States and Russia it will not tolerate Kurdish territorial gains by Kurdish militia close to its frontiers in north-western Syria, two senior officials said.

"This is clear cut for us and there is no joking about it," one official said of the possibility of Syrian Kurdish militia crossing the Euphrates to extend control along Turkish borders from Iraq's Kurdistan region towards the Mediterranean coast.

Turkey fears advances by Kurdish YPG militia, backed by its PYD political wing, on the Syrian side of its 900 km (560-mile) border will fuel separatist ambitions among Kurds in its own southeastern territories. But Washington has supported YPG fighters as an effective force in combating Islamic State.

"The PYD has been getting closer with both the United States and Russia of late. We view the PYD as a terrorist group and we want all countries to consider the consequences of their cooperation," one of the Turkish officials said.

Turkey suspects Russia, which launched air strikes in Syria two weeks ago, has also been lending support to the YPG and PYD.

Masked members of YDG-H, youth wing of the outlawed Kurdistan Workers Party (PKK), sit next to their weapons in Silvan, near the southeastern city of Diyarbakir, Turkey, August 17, 2015. Sertac Kayar/Reuters "With support from Russia, the PYD is trying to capture land between Jarablus and Azaz, going west of the Euphrates. We will never accept this," the official said.

He said Turkey had raised its concerns at high level meetings with the US, European Union and Russia.

The officials did not say what action, if any, Turkey might take if YPG forces crossed the Euphrates. Ankara has carried out air strikes against Turkish Kurdistan Workers Party (PKK) rebels based in the mountains of northern Iraq; but attacks on Kurds in Syria would be far riskier, bringing Ankara into possible conflict both with US and Russian air forces.

The YPG said on Monday it had joined forces with Arab rebels and that their new alliance has been promised fresh weapon supplies by the United States for an assault on Islamic State forces in what is effectively their capital, Raqqa.

Turkey has accused the Kurdish militia of pursuing "demographic change" in northern Syria by forcibly displacing Turkmen and Arab communities. Ankara fears ultimately the creation of an independent Kurdish state occupying contiguous territories currently belonging to Iraq, Syria and Turkey.

Amnesty International on Tuesday accused the YPG, which has seized swathes of northern Syria from Islamic State this year, of committing war crimes by driving out thousands of non-Kurdish civilians and destroying their homes.

The Kurds, who have emerged as the US-led coalition's most capable partner in Syria against Islamic State on the ground, deny such accusations. They say those who left areas they seized did so to escape fighting and are welcome to return.

Over 40,000 people have been killed in a Kurdish insurgency in Turkey since 1984. The collapse of a ceasefire in July has brought a sharp increase in conflict between security forces and Kurdistan Workers Party (PKK) fighters.