BEIRUT (Reuters) - The powerful Syrian Kurdish YPG militia and its local allies have drawn up plans for a major attack to seize the final stretch of the Syrian-Turkish border held by Islamic State fighters, a YPG source familiar with the plan said on Thursday.

Such an offensive could deprive Islamic State fighters of a logistical route that has been used by the group to bring in supplies and foreign recruits.

But it could lead to confrontation with Turkey, which is fighting against its own Kurdish insurgents and sees the Syrian Kurds as an enemy.

After a year of military gains aided by U.S.-led air strikes, the Kurds and their allies already control the entire length of Syria’s northeastern Turkish frontier from Iraq to the banks of the Euphrates river, which crosses the border west of the town of Kobani.

Other Syrian insurgent groups control the frontier further west, leaving only around 100 km (60 miles) of border in the hands of Islamic State fighters, running from the town of Jarablus on the bank of the Euphrates west to near the town of Azaz.

But Turkey says it will not allow the Syrian Kurds to move west of the Euphrates.

The source confirmed a report on Kurdish news website Xeber24 which cited a senior YPG leader saying the plan includes crossing the Euphrates to attack the Islamic State-held towns of Jarablus and Manbij, in addition to Azaz, which is held by other insurgent groups.

The source did not give a planned date, but said a Jan. 29 date mentioned in the Xeber24 report might not be accurate.

The YPG has been the most important partner on the ground of a U.S.-led air campaign against Islamic State, and is a major component of an alliance formed last year called the Syria Democratic Forces, which also includes Arab and other armed groups. The alliance is quietly backed by Washington, even as its NATO ally in the region, Turkey, is hostile.

The political party affiliated with the YPG, the PYD, has been excluded from Syria peace talks the United Nations plans to hold in Geneva on Friday. The PYD and its allies say their exclusion undermines the process and have blamed Turkey.

Ankara fears further expansion by the YPG will fuel separatist sentiment among its own Kurdish minority. It views the Syrian Kurdish PYD as a terrorist group because of its affiliation to Turkish Kurdish militants.

The United States and Turkey have for months been discussing a joint military plan to drive Islamic State from the border, but there has been little sign of it on the ground.

The border area is being fought over by several sides in the complex, multi-sided civil war that has killed more than 250,000 people and driven more than 10 million from their homes.

At the western end of the Islamic State-held stretch of frontier, Syrian insurgents backed by Turkey have been fighting Islamic State near Azaz in a to-and-fro battle that has not yielded major shifts, said Rami Abdulrahman, director of the Syrian Observatory Human Rights.

Tensions between the YPG and its allies on one hand and other insurgent groups backed by Turkey on the other have spilled into conflict near Azaz in the last three months.

Separately, the Syrian army and allied militia, supported by Russian air strikes, are meanwhile edging closer to the Islamic State-held town of al-Bab, some 50 km (30 miles) southwest of Manbij in the Aleppo area.