Taliban fighters have seized a military hilltop site in Kunduz, reinforcing their stronghold on the northern Afghan city.

The Taliban established their preoccupation in Kunduz on Monday; their biggest military gain since they lost power in 2001.This therefore leaves the airport as the army's last stronghold.

The capture of the Bala Hisar fortress came despite efforts by government reinforcements, backed by Nato airstrikes and special forces, to retake the city.

The Taliban had blockaded the Bala Hisar fortress for two days. Hundreds of Afghan security personnel were stationed there, and their fate was not clear.

Late on Tuesday the Taliban tried to capture the airport as well, but two US air strikes halted their advance.

Clashes went on through the night around Kunduz, and the provincial hospital was reportedly struggling to cope with the number of casualties.

Kunduz is one of Afghanistan's largest cities, and is strategically important as a transport hub for the north of the country.

Militant violence has increased across Afghanistan since Nato ended its combat mission in Afghanistan in December, leaving a 13,000-strong residual force used for training and counter-terrorism operations.