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LASHKAR GAH, March 27 (Reuters) - Disguised as an Afghan soldier, a suicide bomber blew himself up in front of a police headquarters and killed at least four policemen in southern Afghanistan on Tuesday, witnesses said.

Several other people were wounded in the attack in Lashkar Gah, the capital of Helmand province, the main drug-producing region of Afghanistan, the world's leading heroin producer.

NATO forces have launched a major offensive in Helmand as spring heralds the return of the fighting season. NATO and Afghan forces have killed dozens of Taliban fighters in battles in recent days.

Last year was the bloodiest since the Taliban's ouster in 2001, with 4,000 people dying.

The rebels have promised to step up suicide attacks as part of a return to conventional guerrilla attacks this year.

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