At least 20 people were killed and 25 wounded when a Taliban suicide bomber blew himself up trying to enter a police building in Afghanistan's capital Kabul, a senior police source has told Al Jazeera.

The area was sealed off by police as ambulances rushed to the scene of what witnesses described as a powerful blast on Monday.

Al Jazeera's Jennifer Glasse, reporting from Kabul, said the attack happened at about 2pm local time.

"The suicide bomber blew himself up in front of a civil order police building in the centre of the city," she said.

"The attack was five days before another scheduled round of peace talks in the presence of Afghan, Pakistan, Chinese and US officials."

The Taliban claimed responsibility for the attack in a statement that said 40 police officers had been killed or wounded. The group often makes exaggerated casualty claims for its attacks.

Kabul was hit by a series of suicide attacks last month, including one that killed seven journalists from a private television station, as the Taliban stepped up its campaign against the Western-backed government.

The attacks coincided with renewed efforts to revive a peace process with the armed group that stalled last year.