KABUL (Reuters) - A suicide bomber in the Afghan capital Kabul killed as many as eight people and wounded many others, most of whom appeared to be workers leaving their offices at the start of the evening rush hour, witnesses and officials said on Tuesday.

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Islamic State claimed responsibility for the attack in Wazir Akbar Khan, a heavily fortified area of the city which houses numerous foreign embassies and government buildings.

A Reuters television team counted eight people who appeared to have been killed, besides several wounded at the scene, which was shrouded in smoke from the explosion. All the casualties appeared to be Afghan civilians.

A public health official said three dead and 15 wounded had been taken to city hospitals but a security official, who spoke on condition of anonymity, said at least seven people had been killed and 21 wounded.

“The scene of the attack was covered in smoke and dust but I saw seven or eight vehicles loaded with dead and injured people coming out of the area,” said Ali Nazari, the manager of a nearby travel agency.

The explosion came at a time of heightened security in Kabul following a series of attacks in October, including one on a Shi’ite mosque in the city that killed more than 50 people and a separate attack on an army training facility that killed at least 15 soldiers.

Security had already been stepped up considerably following a devastating truck bomb attack outside the German embassy on May 31 that killed at least 150 people, with a series of barriers and checkpoints across the central area of the city.

But a Kabul police spokesman said the blast appeared to have been caused by a suicide bomber on foot.

The target of the attack was not immediately clear but as well as foreign embassies, the area contains a defense ministry department dealing with foreign relations and several private businesses, including a barber and a travel agency.

Afghan and Western security officials have said in recent days that they expected more attacks in Kabul in response to pressure being exerted on the Taliban and other insurgent groups by increased U.S. air strikes.

In recent months, the U.S. has stepped up its campaign of air strikes as part of a new strategy aimed at reversing gains made by the Taliban and forcing the insurgents to seek peace talks with the Afghan government.

Western officials say the campaign has inflicted significant casualties on Taliban fighters in the provinces but say the battlefield losses may push the insurgents to attack Kabul and other high profile targets.