The Defense Ministry said Tuesday Nicaraguan rebels ambushed a military jeep and a truck carrying civilians, killing 12 people. It was the deadliest attack since a cease-fire pact was signed in March. Seven civilians and five military men were killed in the raid Monday in Buenos Aires, a village about 100 miles east of Managua, said a military spokeswoman, speaking on condition of anonymity. The spokeswoman said 10 other people - two civilians and eight soldiers - were injured when the U.S.-backed Contras attacked the vehicles with an anti-tank mine, rifle fire and rocket-propelled grenades.