A suicide bomber attacked a government office in Pakistan on Tuesday, killing at least 21 people and wounding more than 70, officials said.

A Pakistani Taliban faction claimed responsibility for the attack on a branch of the National Database and Registration Authority (Nadra), which issues government ID cards, in the northwestern city of Mardan in Khyber Pakhtunkhwa province.

"A suicide bomber riding an explosives-laden motorcycle hit the Nadra office in Mardan where a large number of people were standing in queues," police officer Naeem Khan told Reuters.

At least 21 people were killed and 78 were wounded, 28 of them critically, said Bilal Ahmad Faizi, spokesman for Rescue 1122 emergency responders.

A Nadra employee, Mohammad Tariq, was inside when he heard the huge blast outside the office premises.

"We are still inside the office, and the police and rescue workers are busy in their work," he said.

Jamaat ur Ahrar, a faction of the Pakistani Taliban that is fighting to overthrow the government and establish hard-line Islamist rule, claimed responsibility.

Spokesman Ehsanullah Ehsan said the office was part of the "heathen Pakistan state" and therefore a legitimate target.

"God willing, we will target all Pakistani organisations that are either directly or indirectly a part of this war," Ehsan said in a statement.

The Pakistani Taliban once controlled swathes of remote territory in the northwest, but a series of military offensives that began in 2009 has pushed them back into a few pockets.

There have been fewer militant attacks in towns and cities over the past year or so, compared with several years ago, but the Taliban remain a potent threat.