The weapon's store raid is the latest ambush and attack on security forces and has rekindled memories of the 1975-1992 civil war.
The assault, which took place on Monday, forced hundreds to flee the central town of Savane and interrupted trains on the nearby Sena line, the southern African nation's sole coal export route, said a spokesperson for Brazil's mining firm Vale.
Local newspapers, including the O Pais daily, said guerrillas from the opposition Renamo party were behind the attack. A Renamo spokesperson said the party was investigating the incident and did not claim responsibility.
The defence ministry declined to comment until it had gathered more information.
Relations between Renamo, founded around independence in 1975 with the backing of white-ruled Rhodesia and apartheid South Africa and the ruling, formerly Marxist party, Frelimo, have become increasingly tense.
Two months ago, Renamo fighters killed four police officers in an attack on a police station and a few days later three civilians were killed when gunmen ambushed vehicles on the main north-south highway, a common Renamo tactic during the civil war.
Analysts say Renamo, which has vowed to disrupt local elections this year and a presidential poll in 2014, has at most 1 000 armed men under its command but poses no significant military or political threat to Frelimo.
Many Mozambicans fear even the slightest violence could upset the political stability that has underpinned an unprecedented economic boom based on foreign investment in coal mining and natural gas exploration.
The off-shore Rovuma field is believed to hold enough gas to supply Germany, Britain, France and Italy for 15 years, and mining giants Vale and Rio Tinto have invested nearly $10-billion in mines in Tete province, home to some of the world's largest untapped coal deposits. – Reuters