Updated

Gunmen killed two people in attacks on Mozambique's main north-south highway early on Friday, raising fears of heightened political unrest that could jeopardise a resource-driven economic boom.

In three separate incidents, three vehicles were shot up on the EN1 road in central Sofala province, according to national police spokesman Pedro Cossa, who blamed the main political opposition and ex-rebels Renamo.

"Renamo attacked vehicles in Machanga district which resulted in two deaths, five wounded and the torching of two trucks," Cossa said.

The attacks come just days after former civil war rebel movement Renamo vowed to block the highway and cripple a crucial rail link to coal fields.

Brazilian giant Vale and Anglo-Australian Rio Tinto both use the railway to export coal from Tete province in the northwest through Beira on the Indian Ocean.

No attacks on the railway have so far been reported.

Prime Minister Alberto Vaquina called the latest violence "regrettable".

"People died whose only crime was to use the road while earning their living," he said in comments broadcast on state-controlled Radio Mozambique.

Until 1992, Renamo, which was founded as an anti-communist group, and the Frelimo-led government fought a brutal 16-year civil war, which resulted in the deaths of around one million Mozambicans.

Renamo's support base has since ebbed away, while its leaders have been omitted from the spoils of the country's economic boom.

Six rounds of talks with the communist-rooted Frelimo government about Renamo's demands for more representation in the armed forces and a cut of coal and gas revenues have failed, raising tensions further.

In one attack Friday, a truck was shot at 70 kilometres (43 miles) from the town of Muxungwe, Radio Mozambique reported.

"As we were driving they started shooting from the bush... they shot me," Monica Malote, a hitchhiker who had obtained a lift in the truck, told the station.

In a second incident on the same stretch of the road, a passenger bus came under fire, a local official said.

"Renamo men attacked a bus... They fired three shots. Luckily there were no deaths," Arnaldo Machohe, a district administrator, told Radio Mozambique.

Details of the third attack involving another truck were not immediately clear.

Heavy security and slower than normal truck and bus traffic was reported in the area.

Asked about the attacks on Friday, a Renamo spokesman said he "did not have enough information" to claim or deny responsibility.

"The alert we gave at the press conference was to prevent this type of situation from coming about," Fernando Mazanga told AFP, referring to the attacks.

Police said the Renamo official who issued that warning Wednesday, Jeronimo Malagueta, had since been detained.

Renamo's previous threats of violence had largely been discounted as a strategy to extract concessions from the Frelimo government.

But since Renamo civil war general Afonso Dhlakama set up a military base in the Mozambican bush in October 2012, threats have been matched by a series of attacks.

In April at least two people died in an attack on a bus and truck and in another clash in the area, four police officers and a Renamo fighter died.

On Monday gunmen killed seven soldiers in a raid on an armoury, Cossa said, updating an earlier toll. The government blamed the attack on Renamo.