A man dressed in what was described as a Contra uniform stopped a bus in eastern Nicaragua, forced the passengers out and killed six of them with a submachine gun, according to government reports. Pro-government newspapers said the attack occurred in Camoapa, 54 miles east of Managua. Four more people were killed and one man was wounded when the U.S.-backed rebels attacked the village of Rancho Grande, in eastern Chontales province, the Voice of Nicaragua government radio said. No details on either attack were immediately available. Only scattered skirmishes have been reported since March, 1988, when a cease-fire was signed by the Contras and Sandinistas.