Mozambique's former rebels on Wednesday vowed to block the country's main highway and a crucial rail link to coal fields, dramatically ramping up its standoff with the government.

"Starting Thursday, June 20, 2013, actions will be taken to weaken the logistics of those who make Mozambicans suffer," Renamo's information chief, Jeronimo Malagueta told reporters in Maputo.

Until 1992, Renamo fought a bitter 16-year civil war with Frelimo, which now forms the government.

Mozambique's new found energy and mineral wealth has rekindled animosity between the two parties, with Renamo accusing their foes of hording wealth.

Renamo also accuses the government of amassing military personnel around the bush-camp where its leader, Afonso Dhlakama, has been living since late last year.

Malagueta said the opposition's actions would target infrastructure in an offensive across the central Sofala province, Renamo's traditional stronghold.

One of the targets of Thursday's promised offensive is the only railway line transporting coal between Moatize and the coastal city of Beira.

"Renamo will... paralyse the movement of trains of the railway between Beira-Moatize and Beira-Morromeu," Malagueta warned.

The group also said it intended to block traffic on the main EN1 road which links northern and southern Mozambique.

"Renamo will position itself to stop the circulation of vehicles transporting people and goods, because the government uses these vehicles to transport arms and plain clothed soldiers."

"The government doesn't give us any other option. Renamo is encircled. We are going to lose our leader," said Malagueta.

Renamo's declaration comes after Mozambique's government fingered the ex-guerrillas for a raid on a munitions compound in the central Sofala province in the early hours of Monday morning. At last five soldiers died in the attack.

It is not clear how many weapons the attackers managed to make away with during the raid.

"The attack on the depot of Savane has nothing to do with Renamo's forces of defence and security," said Malagueta.

He suggested instead that "general discontent... might have driven other Mozambicans to resort to arms to dislodge this government."

While it threatened a military offensive, Renamo said it still intends to turn up to another round of political dialogue planned with the Frelimo-led government next Monday.

Six previous rounds of talks have failed to yield a breakthrough on any of the major issues on the table, including Renamo's threat to boycott upcoming polls if the election law is not amended.

Foreign coal companies said they were monitoring the situation closely.

Brazil's Vale and Australia's Rio Tinto rely on the "Sena" rail line to transport coal from the resource-rich northeastern Tete province to the coastal town of Beira.

"Up until now we are monitoring. We have had no order to stop," said Vale spokeswoman Acucena Paul.

"If we heard of people stopping trains, then it would make sense to take out our people, but up until now there is nothing."

Rio Tinto said it was "aware of the situation".

"We are gathering information and monitoring events."

Meanwhile embassies in Maputo warned nationals to avoid using parts of the EN1.

The French embassy advised against travel between the Save River and the town of Muxungue "unless absolutely necessary".