The Renamo opposition party, a former guerrilla organization, threatened on Wednesday to paralyze the only railway line out of Mozambique's vast coalfields in an effort to hurt the government.
Renamo information chief Jeronimo Malagueta told a news conference in Maputo that disruption of the Sena line connecting the northwest region of Tete to the Indian Ocean port of Beira would last "as long as it takes". Renamo would "paralyze the movement of trains", he added.
"From Thursday June 20 we will take action to make the logistics of the country fragile," he said, without providing details. The line is used predominantly by Brazil's Vale and London-listed Rio Tinto.
Malaqueta did not clarify the group's objectives but the threat marks an escalation in its tensions with Frelimo, the former Marxist party that has run Mozambique since the end of a 1975-1992 civil war.
Renamo, founded around independence with the help of white-ruled Rhodesia and apartheid South Africa, accuses Frelimo of exercising a stranglehold over politics and the economy and stacking the election commission to ensure victory in a presidential vote next year.
The Sena line runs through "Renamo country" and skirts the group's semi-militarized stronghold in the Gorongosa Mountains, where it is believed to have around 1,000 men under arms. The line was frequently blown up during the civil war.
Renamo spokesman Fernando Mavanga said the group's plans were "military secrets" and justified action against the coal corridor railway because it serves as an economic lifeline for Frelimo.
Government officials did not respond to requests for comment.
Incidents in the last two months have rekindled memories of the civil war, in which a million people died and 5 million were displaced.
Seven soldiers were killed on Monday when gunmen raided a weapons depot near the Sena line terminus and in April Renamo fighters killed four policemen in an attack on a police station in the same region.
A few days later three civilians were killed when gunmen ambushed vehicles on the former Portuguese colony's main north-south highway, a common Renamo tactic during the civil war.
Renamo denied involvement in the weapons depot assault and a security source said it could have been the work of a splinter group. The same source said government troops had been drafted in on Tuesday to bolster security for the railway.
Analysts say Renamo would be no match for the Mozambique army but an outbreak of fighting could derail an economic boom based on foreign investment in the coal mines around Tete, home to some of the world's largest untapped reserves.
Vale, which is investing $4 billion in its Moatize coal mines near Tete, as well upgrading the Sena line and other infrastructure, declined to comment.
State railways spokesman Alves Cumbe said he was unaware of any specific threat.
"We will only stop operations if we receive orders from the government. If we are told of a security risk, we will stop. Otherwise, we will continue. We have heard nothing," he said.
As well as coal, Mozambique is attracting billions of dollars into developing its offshore natural gas fields, which are thought to hold enough energy to supply western Europe for at least 15 years.