The Taliban has told Al Jazeera it was responsible for a suicide bombing on a police training centre in Afghanistan's Uruzgan province that left 21 people dead.

Officers at the reserve unit in the provincial capital, Tirin Kot, were getting ready for morning exercises at 9.30am [0500GMT] on Monday when the bomber detonated explosives strapped to his body, Juma Gul Himat, Uruzgan's provincial police chief, said.

Many were killed instantly and several later died in hospital, authorities said.

Another 20 police officers were wounded, some of them critically.

The attacker was disguised in a police uniform and had managed to infiltrate the group at the centre, officials said.

Afghan security forces, along with US and Nato-led troops, are struggling to tackle resurgent Taliban fighters who have stepped up suicide attacks and roadside bombings across the country.

'Easy target'

Al Jazeera's Teresa Bo, reporting from Kabul, said the police were an easy target for the Taliban, and that Uruzgan was a centre for the group.

"They are not as well equipped as the foreign troops here, so they are chosen repeatedly for attacks like this one," she said.

"This is an attempt by the Taliban to destabilise the situation, to make a claim that they have control over some specific areas around the country.

"This is one of the main reasons why the independent electoral commission here has postponed the elections until August 20."

In 2008, nearly 870 police officers died in attacks by anti-government fighters, according to a tally of figures collected by The Associated Press news agency.

During the past three years the Taliban have gained greater control of large swathes of the south of the country, following their initial defeat after a US invasion in 2001.