Explosions rocked central Kabul for five hours after Taliban gunmen launched a major suicide and gun attack centred on a compound of the International Organization for Migration, an aid agency.

There are still conflicting reports on the number of casualties in the attack on Friday, but Al Jazeera has learned that at least one police officer was killed, alongside four gunmen, as security forces hunted down the attackers, with prolonged bursts of gunfire and grenade blasts heard across the Afghan capital.

One of the gunmen reportedly died by blowing himself up, injuring three security personnel who were hunting him, Al Jazeera's Qais Azimy, reporting from Kabul, said.

A United Nations building and several other official premises were caught up in the coordinated assault that started when a suicide car bomb sent a plume of dark smoke into the sky.

At least 18 people were also injured, including four UN staffers and four Nepalese guards, Al Jazeera's Azimy said.

The Taliban, fighting to expel Western forces and establish Islamic rule, claimed responsibility, saying a compound used by the US Central Intelligence Agency (CIA), had been targeted, our correspondent said.

Azimy also quoted the Taliban as saying that "they are proud" to have carried the attack in the area, which is under high security.

"It's a show to the people in Afghanistan that they are strong," our correspondent said.

Concern is mounting about how the 352,000 members of the Afghan security forces will cope with the militants after most foreign NATO-led combat troops leave by the end of next year.

Kabul police chief Ayoub Salangi said four attackers had entered a UN compound.

"Our security forces have already killed two of them and two are still on the second floor and fighting with Afghan security forces," Salangi said.

Four blasts

There was no information about anyone who had been inside the compound at the time of the attack.

There were at least four large blasts and exchanges of fire reported between the attackers and Afghan forces, supported by Norwegian special forces, at 6:20 pm (1350 GMT), witnesses said.

The first blast was a suicide car-bomb blast at about 4 pm (1130 GMT) near a main intersection, said Kabul police chief spokesman Hashmatullah Stanikzai.

A Taliban spokesman, Zabihullah Mujahid, speaking to Reuters by telephone, separately claimed responsibility.

Shooting erupted after the first bomb, with more blasts beginning about 30 minutes later.

Insurgent attacks against civilians, government workers and Afghan security forces have increased in recent weeks as the Taliban, toppled by a US-led force in 2001, exert increasing pressure on the Afghan government.

Fifteen people, including six Americans, were killed on May 16 in a suicide bombing by the Hezb-i Islami rebel group, which is allied with the Taliban.

Last year, more than a dozen people were killed during a Taliban attack in Kabul which started with coordinated suicide attacks and led to an 18-hour long siege.