"The United States is deeply concerned by escalating violence between armed members of the opposition party Renamo and the Mozambican armed forces," U.S. State Department spokeswoman Jen Psaki said in a statement.
The BBC reported the opposition Renamo movement abandoned a 1992 peace agreement last week after national forces attacked the central Mozambique headquarters of leader Afonso Dhlakam.
Psaki said Washington was concerned by the use of violence for political gain, calling on all parties to the violence to end their hostilities.
"The United States continues to encourage an environment of open political space to promote the full participation by all parties in democratic institutions and processes," she said.
Nearly 1 million people were killed during Mozambique's civil war that erupted after it gained independence from Portugal in 1975. Civil war ended in 1992 and the country held its first multi-party election two years later.