Zimbabwe’s deputy foreign minister Christopher Mutsvangwa told the BBC that he would not countenance a return to civil war in Mozambique.
The Renamo movement, thought to have about 1,000 fighters as well as 51 MPs, ended the peace accord on Monday.
Mozambique’s 1976-1992 civil war led to about one million deaths.
Mr Mutsvangwa said Renamo should rejoin the political process, not threaten regional stability.
He told the BBC’s Focus on Africa radio programme that Renamo leader Afonso Dhlakama should “never, never” return to violence.
“Southern Africa will not countenance this. We simply do not need this in this region at this juncture,” he said.
He added that regional body Sadc (Southern African Development Community) would consider sending troops to help the government if the security situation deteriorated.
“It will be misguided for Renamo to bring instability and expect Zimbabwe to watch,” he said.
Renamo pulled out of the peace accord after government forces captured Mr Dhlakama’s base in the Gorongosa mountains in central Mozambique on Monday, forcing him to flee.
Rebel fighters retaliated by opening fire on a police station in the town of Maringue, about 35km (20 miles) from the base, state-run Radio Mozambique reported.
The US has urged the two sides to “step back from the brink”.
“We are encouraging the two parties to take visible and decisive steps to de-escalate the current tense environment,” said US state department spokeswoman Marie Harf.
Mozambique’s economy has been booming in recent years, with the discovery of a major off-shore gas field in 2011.
Mozambican mediator Lourenco do Rosario said he held talks with Renamo to ease tension.
He said Renamo leaders had told him they did not want to return to war.
Renamo was supported by South Africa’s former white-minority regime during the civil war that raged after Mozambique’s independence in 1975.
After Robert Mugabe came to power in Zimbabwe, he backed Mozambique’s Frelimo government.
Renamo has contested elections since the end of the civil war, but has failed to dislodge Frelimo from power.
Renamo’s 51 MPs have not withdrawn from parliament, despite its decision to pull out of the peace accord.
Mozambique is due to hold local elections in November, and presidential and parliamentary elections next year.
Renamo has complained that the Frelimo government is determined to hold on to power and has failed to create conditions for free and fair elections.
Frelimo denies the allegations and says Renamo does not have support among voters.