KABUL, Afghanistan — A suicide bomber rammed his explosives-laden car into a minibus carrying Afghan soldiers south of Kabul on Wednesday, killing at least 13 people and wounding 20 others, officials said.

Meanwhile, Afghan forces clashed with Taliban who had blocked a main highway in the south, killing 10 militants, an official said.

The Kabul blast happened on the last day of Defense Secretary Robert Gates' two-day visit to Afghanistan, but it was not immediately clear if he was still in the country at the time. The Taliban claimed responsibility for the attack.

The suicide bomber's car struck a minibus full of soldiers in the Chihulsutoon area south of Kabul, said Aziz Ahmad, an Afghan army officer at the site of the blast.

Six soldiers and seven civilians were killed in the attack, and seven other soldiers were wounded, said Gen. Mohammad Zahir Azimi, a Defense Ministry spokesman.

At least 13 civilians also were wounded in the attack, said Abdullah Fahim, a spokesman for the Health Ministry. Four children were among those killed, Fahim said.

Purported Taliban spokesman Zabiullah Mujaheed claimed responsibility for the blast in a text message sent to an Associated Press reporter in neighboring Pakistan. Mujaheed identified the bomber as Abdul Rahman, from eastern Khost province.

The mangled frame of the minibus lay on the side of the road as the wounded were whisked to hospitals.

The blast was third suicide attack in the city in the last eight days. It followed a similar attack Tuesday against a NATO convoy that wounded 22 civilians.

Mohammad Amin, who runs a bakery near the blast site, said two of his employees were wounded by flying glass.

"Every day, this bus stops in front of my bakery to take employees of the Defense Ministry," Amin said. "Suddenly today, a very strong explosion hit the bus."

Mohammad Ashraf, 13, was praying inside a mosque when the flying shrapnel and glass cut through his flesh, said his father, Mohammad Akram.

"My other 8-year-old son was also wounded in the same mosque," Akram said.

There has been a series of attacks in recent months on buses carrying Afghan security forces as they commute to work in the morning.

On Sept. 29, a suicide bomber blew himself up inside an army bus in Kabul, killing 28 soldiers and two civilians. In June, another bomb ripped through a bus carrying police instructors in Kabul, killing 35 people.

Militants have launched more than 133 suicide attacks this year — a record number. At least 6,200 people have died in insurgency-related violence in 2007, also a record, according to an Associated Press tally of figures from Afghan and Western officials.

Suicide attacks frequently target international and Afghan security forces, but most of the casualties are civilian passers-by.

Separately, an Afghan army patrol clashed with Taliban fighters in southern Zabul province's Shah Joy district on Wednesday after the militants set up a checkpoint on the main highway linking Kabul to the country's south, said Abdul Raziq, a provincial Afghan army commander.

Ten militants were killed, and soldiers recovered 12 motorbikes and weapons from alongside the bodies, Raziq said.

In Helmand province, U.S.-led coalition troops killed several Taliban militants during raids on compounds in Garmser district, the coalition said.

The troops "targeted an individual believed to be associated with weapons smuggling operations in the province," it said. "While performing a search of one of the compounds, coalition forces killed several armed militants who posed an imminent threat."

Also in the south, an explosion struck a patrol of NATO-led troops on Tuesday, leaving one soldier dead and two others wounded, the alliance said. The soldiers' wounds were not life-threatening, the statement said.

NATO did not disclose the nationalities of the dead and wounded soldiers or the exact location of the attack.

Associated Press writers Rahim Faiez in Kabul and Noor Khan in Kandahar contributed to this report.