At least 21 people died when gunmen armed with grenades and Kalashnikovs attacked a university in Pakistan on Wednesday as security forces moved in under thick fog to halt the bloodshed.

The assault was claimed by a Pakistani Taliban faction but branded "un-Islamic" by the umbrella group's leadership, which vowed to hunt down those responsible.

The number of dead climbed rapidly after armed men stormed Bacha Khan University in the northwestern town of Charsadda in the latest outrage to hit the militant-infested region.