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Several suspects have been arrested for a suicide attack that killed six U.S. troops when an explosives-packed minibus blew up at the entrance of a joint NATO-Afghan base in southern Afghanistan, officials said Monday.

NATO spokesman Brig. Gen. Josef Blotz said that several arrests had been made Sunday night for the blast, which was the deadliest attack on coalition troops this month.

"Individuals believed to be involved in yesterday's attack have been arrested by Afghan and coalition forces," Blotz said at a news conference, adding that no shots were fired as the suspects were taken into custody.

NATO has declined to identify the victims' nationalities, but an Afghan army official in southern Afghanistan said on Monday that the six were Americans. He spoke on condition of anonymity because he was not authorized to speak about NATO casualties.

He said the confirmation came in an official report about the attack.

NATO has claimed improvements in security after months of raids, patrols and strikes on insurgents in Kandahar province, but Sunday's attack shows the area is still far from safe.

The assault came just days ahead of a major White House review of its Afghan strategy following President Barack 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 of irrigation canals and grape vineyards that the Taliban have used as a staging area for attacks on 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.

More than 680 international troops have been killed so far this year, well above the 502 killed in 2009. The last attack to kill that many NATO troops happened Nov. 29, when an Afghan policeman turned his gun on his American trainers in the east, killing six of them before he himself was shot dead. The Taliban claimed that they had sent him to join the police as a sleeper agent.

Two weeks before that attack, insurgents killed five U.S. soldiers in eastern Afghanistan.

The level of ongoing fighting and the mounting death toll will be key to the Obama administration's December review. The president has committed to begin withdrawing U.S. forces in June 2011, but the feasibility of that goal will depend greatly on whether commanders believe last year's surge has reined in violence to the point that Afghan forces can start taking the lead.

In Kabul, meanwhile, a group of about 100 recently elected parliamentarians signed a letter urging President Hamid Karzai to convene the new parliament by Dec. 19 in order to end uncertainty about the legislature following an election plagued by accusations of fraud.

Parliament is currently in its winter recess, which typically ends around mid-January. The lawmakers urged Karzai to start the new session next week to show that pending Supreme Court cases will not block legislature's work.

Afghanistan's attorney general has called on the court to annul the elections, citing evidence that high-level officials were involved in cooking the results. Election officials, meanwhile, have said that neither the attorney general nor the Supreme Court have the authority to change the results.

Karzai has said that he is not planning to delay the opening session and his spokesman argued Monday that the disagreements over how to resolve election disputes will not spark a crisis.

"What is right now going on between the institutions in Afghanistan is a normal issue in a democratic country," spokesman Waheed Omar said. "There are specific institutions who are working on interpreting the law and we are sure that a legal solution is going to be found."