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LASHKAR GAH, Afghanistan, April 4 (Reuters) - A suicide bomber in southern Afghanistan attacked and killed a police commander on Friday who was responsible for the arrest this week of a mid-level Taliban leader, a provincial police chief said.

Two other policemen and a civilian were killed in the blast in the capital of Helmand province, Lashkar Gah, police said.

The latest violence in Afghanistan came hours after NATO leaders meeting in Bucharest confirmed their "firm and shared long-term commitment" to Afghanistan.

The bomber blew himself up after walking up to a vehicle in which a police officer known as Commander Mareez was travelling with some of his men, said police chief Mohammad Hussain Andiwal.

Mareez was responsible for the capture on Monday of an insurgent commander known as Naqibullah, Andiwal said.

"The Taliban are behind the attack in retaliation for the capture of their commander," Andiwal said.

Three policemen were wounded in the attack, Andiwal said. Several shopkeepers were also hurt, residents said.

Taliban insurgents fighting to oust NATO- and U.S.-led troops and bring down the Western-backed government have vowed to step up their insurgency.

Taliban commander Naqibullah was captured after a shoot-out near Lashkar Gah in which three Taliban were killed. Naqibullah had twice escaped from prison and the Interior Ministry said in February he had been killed in a clash.

With suicide attacks and car bombings on the rise, tension has arisen among Afghanistan's Western backers in recent months over whether countries have been doing enough in the battle against the Taliban.

In a joint declaration issued at the Bucharest summit, the 40 nations of the NATO-led peacekeeping mission said they were committed to the country for the long haul.

Canada pledged on Thursday to keep its troops in Afghanistan after France offered to bolster the NATO force there.

A pledge by French President Nicolas Sarkozy of 700 more troops, coupled with planned contributions by other nations, proved enough for Canada to withdraw a threat to pull its 2,500 troops out of the 47,000-strong NATO-led force.

U.S. President George W. Bush welcomed Sarkozy's pledge and said it would allow some U.S. troops to move from the east to the Afghan south, scene of the worst violence. The United States is sending an extra 3,200 Marines to Afghanistan.

NATO also set a goal of helping train an 80,000-strong Afghan army by 2010, as part of what the 26-nation alliance hopes will allow domestic forces to gradually lead more security operations across the country. (Reporting by Haji Qudous; Editing by Robert Birsel)

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