KABUL, Afghanistan – An explosives-packed minibus blew up at the entrance of a joint NATO-Afghan base in southern Afghanistan on Sunday, killing six NATO troops and two Afghan soldiers as they prepared to head out on patrol.

NATO has claimed improvements in security after months of raids, patrols and strikes on insurgents in Kandahar province, but Sunday’s blast — the deadliest attack on coalition troops this month — shows the area is still far from safe.

The assault comes days ahead of a major White House review of its Afghan strategy following President Obama’s decision last year to send 30,000 American reinforcements in a bid to reverse gains by the Taliban since they were ousted from power in the 2001 U.S.-led invasion.

Afghan officials said Sunday’s suicide attack took place in Kandahar’s Zhari district, where Mullah Mohammad Omar organized the Taliban in the early 1990s. Taliban spokesman Qari Yousuf Ahmadi claimed responsibility for the blast, saying the insurgent group was retaliating for attacks on its fighters in the area in recent months.

U.S. and Afghan forces launched a major operation in September to secure Zhari, a lush farming region that the Taliban have used as a staging area for attacks in nearby Kandahar city and other parts of the south.

Zhari has remained insurgent territory despite five major NATO operations in recent years. In 2006, a Canadian-led force launched a concerted push in Zhari and nearby Panjwai district, driving out the Taliban but at a cost of 28 coalition lives. Months later, the Taliban were back.

In a two-day trip to Afghanistan last week, Defense Secretary Robert Gates said Washington’s year-old surge strategy for the war is working and the goal of pulling U.S. troops out of major combat by 2014 can be met.

“Coalition and Afghan forces are suffering more casualties, but there is no denying that the security climate is improving and that the sacrifices of Afghan and coalition troops are achieving greater safety and security for both our nations,” he said.

At the Howz-E-Madad base in Zhari district, the No. 2 coalition commander in Afghanistan, Lt. Gen. David Rodriguez, told reporters last weekend that he expected the enemy to retaliate after losing “a bunch of terrain they used to control.”

He said insurgents are responding with a “murder and intimidation campaign” to try to stop the progress that coalition forces are making in the south.

In Sunday’s attack, a suicide bomber drove a minibus into the entrance of the coalition base just as troops were preparing vehicles to move out on a morning patrol, said Gen. Abdul Hamid, the Afghan army chief for Kandahar province.

NATO said that the six service members had been killed in an insurgent attack, though it did not disclose their nationality. Most coalition forces in the south are American.

More than 680 international troops have been killed so far this year, well above the 502 killed in the whole of 2009. The last attack to kill that many NATO troops happened Nov. 29, when an Afghan police officer turned his gun on his American trainers in the east, killing six.