KABUL - At least five suicide bombers attacked a major hotel frequented by Westerners in Afghanistan's capital late on Tuesday, a senior police officer said, with fighting continuing as police cleared the building.

Witnesses heard four blasts over the course of about two hours, with intermittent bursts of gunfire heard during the late-night attack on the Intercontinental Hotel, one of two main hotels used by foreigners in Kabul.

Mohammad Zahir, the head of the Kabul police crime unit, said three police officers had been wounded as they cleared the hotel on the city's western outskirts.

"The clearing of the Intercontinental is still going on. It's not over yet," Zahir said.

The attack came the night before the start of a conference about the gradual transition of civil and military responsibility from foreign forces to Afghans.

The hotel was not one of the venues to be used by the conference or its delegates, an Afghan government official said.

Taliban spokesman Zabihullah Mujahid said several fighters from the Islamist group had attacked the hotel.

Mujahid, who spoke to Reuters by telephone from an undisclosed location, said heavy casualties had been inflicted. The Taliban often exaggerate the number of casualties in attacks against Western and Afghan government targets.

Flares, Tracer Rounds

One blast was heard at the start of the attack and then three more at least an hour later, one of the witnesses said. Bursts of gunfire were heard over the same period and flares lit up the sky over the hotel.

Television footage showed police firing tracer rounds into the air as other officers moved through the hotel. Power was cut in the hotel in the hotel and in surrounding areas after the attack.

A police source said that a wedding party was underway when the attack happened.

The hotel, built on a hillside in western Kabul with heavy fortifications all around it, is often used for conferences and by Westerners visiting the city.

Police threw up roadblocks immediately after the blast, stopping people from approaching the area.

Violence has flared across Afghanistan since the Taliban announced the start of a spring offensive at the beginning of May, although Kabul has been relatively quiet.

The last major attack on a similar hotel used by foreigners was in January 2008, when several Taliban gunmen killed six people in a commando-style attack on the nearby Serena hotel. The Serena has been hit in several random rocket attacks since then.

The increase in violence comes as NATO-led forces prepare to hand security responsibility to Afghans in seven areas from next month, at the start of a gradual transition process that will end with all foreign troops leaving Afghanistan by the end of 2014.

The two-day conference to discuss the transition process was due to begin in a government building in the center of the city on Wednesday.

Violence across Afghanistan in 2010 was already at its worst levels since the Taliban were ousted by US-backed Afghan forces in late 2001.