KABUL: At least six people were killed in a suicide car bomb attack  in the Afghan capital  on Wednesday, officials said, hours after U.S. President Barack Obama left Kabul following an unannounced visit during which he signed a strategic partnership agreement.

The blast, and Obama's visit, came a year after U.S. special forces troops killed al Qaeda leader Osama bin Laden, the architect of the September 11, 2001 attacks, in a raid in neighboring Pakistan.

The Taliban, ousted by U.S.-backed Afghan forces for harboring bin Laden and other militants, quickly claimed responsibility for what it said was a suicide car bomb attack.

Wednesday's attack was the latest in a recent surge of violence after the Taliban announced they had begun their usual "spring offensive", and that they had suspended tentative steps towards peace talks with the United States.

Kabul police chief Ayoub Salangi said six people, a Gurkha guard and five passers-by, were killed in the Kabul attack.

Salangi told Reuters a car bomb exploded on Jalalabad road, the main road out of the capital heading east, where several U.S. military bases and compounds housing Westerners are located.

He said one of those compounds, known as "Green Village", was the target. A plume of black smoke was seen rising from inside the area.

Reuters witnesses heard a second blast in the same area about 90 minutes after the first explosion.

The Interior Ministry earlier said at least two people had been killed , and another police official said an exchange of gunfire followed the first blast.

Afghan media said heavy casualties were feared.

Taliban spokesman Zabihullah Mujahid said: "One of our mujahideen detonated his car in front of a military base."

"Other mujahideen are inside the base fighting. There are very heavy casualties for the enemy," Mujahid told Reuters by telephone from an undisclosed location.

The Taliban often exaggerate accounts of attacks involving foreign troops or Afghan government targets.

A spokesman from NATO headquarters in the country said the coalition was aware of several explosions.

Obama marked the anniversary of bin Laden' s death with a speedy trip to Afghanistan, signing the strategic pact with Karzai and delivering an election-year message to Americans that the increasingly unpopular war is winding down.

Most foreign combat troops are due to leave Afghanistan by the end of 2014 under a plan to hand security responsibility to Afghan forces.

Obama left the country about three hours before the explosion, after making a televised address to Americans from Bagram Air Base north of the capital.

A U.S. embassy warning system urged staff to stay away from windows and take cover after the explosion. The embassy is in the main diplomatic area in the center of the city.

Insurgents staged coordinated attacks in Kabul last month, paralyzing Kabul's centre and diplomatic area for 18 hours.

The Taliban claimed responsibility for those attacks, but U.S. and Afghan officials blamed the militant, al Qaeda-linked Haqqani network.