ISTANBUL — The United States has stepped up its military support for Syrian Kurdish militias fighting the Islamic State, efforts that have angered Turkey, a longtime ally and NATO member, which is now weighing new measures to contain the ambitions of the Kurds, including a buffer zone within Syria.

Ankara sees the Syrian Kurds as a serious national security threat because of their links to Kurdish nationalists in Turkey, who have waged a decades-long insurgency against the Turkish state. So it has looked on with growing concern at the expanding cooperation between the Syrian Kurdish militias and the United States military in the fight against the Islamic State, also known as ISIS or ISIL.

The United States now maintains extensive surveillance over northern Syria with drones and aircraft to help the Kurdish militias, and American Special Forces officers have set up communication links to feed the Kurds intelligence and help them call in airstrikes by the United States-led coalition.

As one of the few fighting groups that can reliably be counted on to fight ISIS rather than the Syrian government of President Bashar al-Assad, the Syrian Kurds have become an ally of growing importance to the United States. The close coordination, begun last year during the fight for Kobani, has grown and evolved in recent months, culminating in the recent rout of ISIS fighters from the strategic town of Tal Abyad, just over the border from Turkey.