(Adds Pakistani engineer shot) LASHKAR GAH, March 27 (Reuters) - A suicide bomber disguised as an Afghan soldier blew himself up in front of a police headquarters and killed at least four policemen in southern Afghanistan on Tuesday, witnesses and officials said. Several other people were wounded in the attack in Lashkar Gah, capital of Helmand province. Helmand is the main drug-producing region of Afghanistan, the world's leading heroin producer. "The bomber blew himself up as police stopped him while he was entering the police headquarters," witness Rahimullah said. Provincial police chief Nabi Mullahkhail, the apparent target of the attack, was inside the compound at the time but was unhurt, police said. NATO forces have launched a major offensive in Helmand as spring marks the end of the relative winter lull in fighting. NATO and Afghan forces have killed dozens of Taliban fighters in battles in recent days. Last year was the bloodiest since the Taliban's overthrow 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. In a rare attack in the relatively safe north, two gunmen on a motorcycle shot dead a Pakistani engineer and wounded two Afghan colleagues, police said.
