The death toll from a car bomb attack on a marketplace in northwestern Pakistan has risen to 21, officials said on Tuesday.

The bomb exploded on Monday in Jamrud, part of the tribal district of Khyber, close to where passengers were waiting to catch buses across the country.

"Four more people died of their wounds at the Hayatabad Medical Complex overnight, raising the death toll in the attack to 21," doctor Sohail Ahmad said.

Khyber district is part of Pakistan's semi-autonomous tribal belt on the Afghan border where the Taliban and other Al-Qaeda-linked groups have strongholds.

Local administration official Jehangir Azam confirmed the new toll and said 43 people were still being treated for injuries.