DAMASCUS —More than 21 pro-government militiamen have been killed in two days of battles with Syrian Kurdish forces in northeast Syria that has widened the Kurds’ control over a city at the border with Turkey, a Syrian Kurdish official said.

Five members of the Kurdish internal security force, the Asayish, were also killed in the fighting in Qamishli, which the official said was the second most violent between the government and Kurdish forces since the Syria conflict began in 2011.

A Reuters witness saw at least 40 members of the pro-government militia surrendering to the Kurdish forces in the city after a battle for a prison where they were holed up on Thursday. Heavy gunfire could be heard in the city.

Syrian army officials could not be reached for comment.

Qamishli is mostly controlled by Kurdish security forces that took control as the crisis spiraled in 2011. Syrian Kurdish groups now control wide areas of northern Syria where they have set up their own government.

The main Kurdish groups and their allies aim to finalize plans within six months for an autonomous political federation in northern Syria, pressing ahead despite the objections of foreign governments that fear Syria’s disintegration.

Kurdish and government forces have mostly avoided confrontation since the start of the Syrian uprising in 2011.

The Kurdish official said the violence was triggered by Syrian government arrests and attacks on civilians.

The Syrian government still controls a few areas in the city center, and its airport.