The agreement between Renamo and the government-dominating Frelimo party brought an end to the civil war between the groups, which started in 1975.
Renamo spokesman Fernando Mazanga told Reuters that "peace is over in the country” and its 51 parliamentarians would meet to decide their strategy.
“He did not immediately spell out whether the opposition movement would be taking up arms again on a national level,”Reuters reported.
The opposition also claimed that government troops had taken over the Renamo base in Sofala province, which followed armed clashes in the area.
The end of the peace accord is expected to alarm investors.
Rio was already eyeing a potential sale of its operations and assets in Mozambique mid-year.
In January it announced $A3 billion write-down on its $4 billion Riversdale Mining acquisition, less than two years after it was completed.
At the time chairman Jan du Plessis said that developing infrastructure in Mozambique proved to be more difficult than anticipated.
Last month Vale awarded WorleyParsons the project management contract to build a port-connecting rail line to its Moatize mine in the country, which would lift coal exports to 18 million tonnes per annum.
