The blast occurred near the town of Vila-Nova in Huambo province when the bus ran over an anti-tank mine, the reports said.
The Lusa news agency said it was the first such incident since Huambo province was recaptured during a government offensive against rebel UNITA forces in October.
The West African state has known little but civil war since winning independence from Portugal in 1975.
The latest upsurge in violence began a year ago when a four-year-old peace accord between the government and UNITA finally collapsed.
Despite some initial setbacks, the government appears to be getting the upper hand, driving UNITA out of its central highland headquarters.
- Reuters - Thomson Reuters Foundation
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