Pillay said she welcomed interim President Catherine Samba-Panza's call for an end to violence in CAR but expressed concern following reports of clashes last week between members of Christian anti-balaka militias and the rival Seleka rebel coalition.
"There is a need to urgently restore security not only in [the nation's capital] Bangui but also in other parts of the country," she said in a statement.
Seleka toppled the civilian government in March. Former President Michel Djotodia, a former Seleka rebel leader, took over after the rebellion, but quickly lost control of the situation.
U.S. Secretary of State John Kerry issued a statement Sunday expressing alarm over the security situation in CAR.
"We are also deeply disturbed by reports that some CAR leaders may be supporting attacks by anti-balaka and Seleka, respectively, instead of working for peace and reconciliation," he said.
Pillay said parts of the country are deserted after clashes erupted last week. Members of anti-balaka have threatened international organizations for sheltering Muslim relatives of their staff and at least 10 people were killed in recent fighting, she said.