BEIRUT, Lebanon — A Kurdish group captured a town in Syria from Islamists fighting in Syria’s civil war on Saturday in a battle that killed 28 fighters, most of them Islamists, a monitoring group reported.

If the Kurds can hold on to the town, Tell Brak, on a highway between the cities of Hasaka and Qamishli, it will be a significant advance in their quest for wider control in the northeast.

Islamist activists said online that fighting was still going on, but the Britain-based opposition monitoring group, the Syrian Observatory for Human Rights, said Kurdish groups known as People’s Protection Units had taken the town.

Syrian Kurds have expanded their sway in the northeast, where they are setting up their own administration, since the revolt against President Bashar al-Assad began three years ago.