Roadside bombs and an insurgent attack claimed the lives of five coalition troops Wednesday, NATO said, while the Taliban took aim at the Afghan intelligence services, killing four people and wounding more than 30 in a pair of attacks, officials said.

The violence follows a surprise visit to Kabul a day earlier by U.S. Vice President Joe Biden, who praised coalition advances made against the insurgency while noting that the gains were "fragile and reversible." Biden left Afghanistan for neighboring Pakistan Wednesday morning.

Roadside bombs have been the most deadliest weapon in the insurgent arsenal against international forces in the course of the Afghan war. NATO said four of its troops were killed in eastern Afghanistan, three of them by a roadside bomb and one in an insurgent attack.

Another was killed by a bomb in the south, the alliance said, bringing the total number of international forces killed so far this year to 16.

Last year was the deadliest of the nearly decade-long war for international forces, with more than 700 killed, compared to just over 500 in 2009.

Wednesday's violence began in Kabul during the morning rush hour, when a suicide bomber on a motorbike struck at a minibus carrying Afghan intelligence service employees. President Hamid Karzai's office initially said four people had been killed in the blast, but later revised the figure downwards to at least two, saying it was based on more complete reports.

The Interior Ministry said one of the two killed was an intelligence service member and the other a civilian, while 32 people were wounded, including six other members of the intelligence service.

The powerful blast struck on a busy road in the western part of the capital, shattering the windows of dozens of houses. The suicide bomber's body lay in the street near the wreckage of his motorbike as police and intelligence officials cordoned off the area.

About an hour later in the troubled eastern province of Kunar, a remote-controlled roadside bomb killed a colonel with the intelligence service and his driver, and wounded two bodyguards, said Abdul Saboor Allahyar, deputy chief of Kunar's provincial police.

The Taliban claimed responsibility for both attacks.

Insurgents often target Afghan security officials, but the nation's capital has been largely spared the worst of the major attacks in the country over the past few months.

The Taliban has proven resilient in the face of the U.S.-led military coalition's nearly decade-long war. Although NATO poured more than 30,000 extra troops into the country last year to pressure the insurgents' traditional strongholds in the south, the Taliban have boosted their operations elsewhere, launching attacks across the north and east.

An extra contingent of 1,400 U.S. Marines are to be deployed in the coming months in the southern province of Helmand, which along with neighboring Kandahar have seen some of the fiercest fighting.

NATO says its campaign so far has had a significant impact. The alliance's spokeswoman, Oana Lungescu, said that "thousands of insurgent leaders have been killed or captured and several thousand fighters have been taken off the battlefield" in the past year.

Lungescu described the guerrilla war as an "industrial strength" insurgency, but disputed recent estimates from military and diplomatic officials at NATO headquarters that placed Taliban forces at up to 25,000 fighters. She called such estimates "highly unreliable," and said focusing on the numerical strength of the Taliban misrepresents gains made by the alliance in the past year.

"There has never been a single reliable source for the size of the insurgency. Numbers referred to a year ago were certainly based on best guess estimates at that time," Lungescu said in e-mailed comments, adding that estimates at the end of 2009 "were anywhere from 25,000 to 35,000."

Afghan Defense Ministry spokesman Gen. Mohammad Zahir Azimi said Wednesday that no official Afghan government figures were available, but he estimated the size of the Taliban was between 25,000 and 35,000 people.