UN chief Ban Ki-moon called on Mozambique's government and the former Renamo rebels to stop the violence and engage in dialogue amid an escalation in violence.
The ex-rebels declared a 1992 peace deal over on Tuesday, although they later reaffirmed they wanted to avoid a return to war.
Ban "calls on all parties to refrain from any act that can threaten the peace and stability that has prevailed during the past 21 years", his office said in a statement.
"He urges them to fully engage in an inclusive dialogue to resolve differences within the established democratic order and to ensure that the country continues to achieve social inclusion and sustainable development for all."
Two days of tit-for-tat violence since the Renamo's announcement has raised fears the country could plunge back into civil war, two decades after the end of one of Africa's bloodiest conflicts in which around a million people were killed.
The violence saw fighters from the Renamo, now an opposition party, attack a police station. Schools were closed in the region around Renamo's military base, in the mountains of central Mozambique.
Monitoring political, security developments
Meanwhile, the French embassy in Maputo on Wednesday advised against travelling to the troubled central Mozambican province of Sofala, the centre of the unrest.
"We advise against any road travel in the province of Sofala, except Beira, unless necessary," said the embassy.
The French embassy said it was working closely with other EU member states and the US embassy to monitor political and security developments in Mozambique.
No foreign nationals have so far been caught up in the violence, according to the statement.
Renamo is demanding a bigger role in the affairs of the country and has staged numerous deadly attacks since returning to the bush. – AFP