BEIRUT, Lebanon — The Turkish government took the extraordinary step on Wednesday of asking the United States to stop Kurdish commanders from diverting their forces from areas of eastern Syria to the fight in Afrin in the west.

The request followed an announcement from the Kurdish forces, which are allied with the United States in the fight against the Islamic State militant group, that they intended to send 1,700 fighters from Deir al-Zour, a strategically important eastern province, to the fight against Turkey in Afrin, a Kurdish enclave.

Ibrahim Kalin, a spokesman for President Recep Tayyip Erdogan of Turkey, said his country had taken “the necessary steps” through official channels and “expected from the U.S. that it should absolutely step in” to prevent the movement of the Kurdish forces from Manbij to Afrin. “This is our most natural right,” Mr. Kalin added.

It was not clear why Mr. Kalin had referred to Manbij, a city at the westernmost point of the Syrian territory held by the Kurds, though it might have been cited as a way station for the troops as they moved toward Afrin.