KABUL SEOUL, South Korea — LOS ANGELES — - A suicide car bomber struck a barrier outside a U.S. base in Kabul on Tuesday, wounding six Afghans and eight American troops hours after gunmen killed four policemen in southern Afghanistan. The car bombing was the latest attack to hit Kabul, coming just over a week after a team of Taliban gunmen and suicide bombers staged an assault that paralyzed the city and left 12 people dead. The violence has underscored fears that militants would try to stage attacks ahead of a key international conference on Afghanistan to be held Thursday in London. The bomber detonated a minivan packed with explosives near Camp Phoenix, a U.S. base.