(Adds details on casualties)

By Sayed Salahuddin

KABUL, Dec 5 (Reuters) - A Taliban suicide bomber slammed his car into a bus filled with Afghan soldiers in Kabul on Wednesday, killing 13 people, the second such attack in as many days around a visit by the U.S. Defense Secretary.

Six soldiers and seven civilians were killed during the morning rush hour attack in Kabul's southern outskirts. Seventeen people, including seven army officers, were wounded, the defense ministry said in a statement.

It was not immediately clear if the civilians were travelling in the bus or caught on the road by the blast, which also caused gas containers in a nearby shop to explode, witnesses said.

Four of the civilian dead were children, a health ministry official said.

Witnesses said ambulances rushed to evacuate the victims. A Taliban spokesman said a member of the resurgent Islamic group carried out the attack.

It was not immediately clear in Gates was in town at the time of the blast. The Pentagon chief flew into the northern Iraqi city of Mosul from Kabul on Wednesday.

NATO CONVOY

Removed from power in 2001, the Taliban rely heavily on suicide raids and roadside bomb attacks as part of their insurgency against the Afghan government and foreign troops stationed in the country.

The worst suicide attack in Kabul hit an army bus in September and killed 28 army personnel.

On Tuesday, a NATO convoy was hit by a similar attack close to Kabul's international airport. There were no casualties among NATO forces, but 22 Afghans were wounded, according to NATO.

More than 10,000 people have been killed in the past two years, the bloodiest period since the Taliban's ouster, and an increasing number of Afghans are frustrated with the lack of progress toward peace.

The blast on Tuesday came during a visit by U.S. Defense Secretary Robert Gates to assess the rising violence. A Taliban spokesman said the militant Islamic group carried out the attack to "welcome" Gates.

Afghanistan's army chief, General Bismillah Khan, asked Gates for more security trainers and equipment to fight the Taliban insurgency.

Gates said the Pentagon was looking for ways to expedite delivery of needed weapons and supplies to Afghanistan. But he stressed other NATO partners in Afghanistan must dedicate more resources to the war effort.

The United States has 26,000 troops in Afghanistan. About half are involved in NATO operations and half on other missions. (Editing by Bill Tarrant)

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