ANKARA (Reuters) - Turkey is not contributing to a U.S.-backed operation by Syrian fighters, some of them Kurdish, against Islamic State in the last tract of territory the group holds near the Turkish border, a Turkish military source said on Wednesday.

Ankara had been informed by Washington about the operation near Manbij, in a region some 40 km (25 miles) from its border, but could not support it because of the involvement of Kurdish YPG militia fighters and because it was beyond the range of artillery stationed in Turkey, the source said.

“Turkey has no contribution to the support that the U.S. gives to the YPG in Syria’s Manbij region. Turkey was informed by the United States about the operation, but any contribution is out of the question,” the source said.

“This region is 40 km from Turkey’s border and therefore it is impossible for Turkey to provide support. It is also out of the question politically for Turkey to give support to a YPG operation,” he said.

U.S. officials told Reuters thousands of fighters, supported by a small number of U.S. special forces, were launching an offensive to capture the crucial swathe of northern Syria that militants have long used as a logistics base.

The operation would be overwhelmingly comprised of Syrian Arabs rather than the Kurdish YPG, who will only represent about a fifth or a sixth of the overall force, the officials said.

But the Syrian Observatory for Human Rights, which monitors the war, said the YPG made up the majority of the fighters taking part in the assault by the Syria Democratic Forces (SDF) alliance, with the initial target of capturing Manbij town.

Turkey, a NATO member, has strongly opposed any further expansion by Syrian Kurdish militia fighters at the frontier. Ankara says the YPG has deep ideological and logistical ties to Kurdish insurgents fighting for autonomy in its own southeast.