The Office of the U.N. High Commissioner for Human Rights said it's received "worrying reports" about the use of tear gas, rubber bullets and birdshot pellets. The OHCHR said "reliable sources" indicated that a number of deaths were linked to the use of tear gas fired by security forces into crowds.
Bahrain has been responding to a largely Shiite uprising against the Sunni-led monarchy. The government was criticized for its crackdown against demonstrators.
"We call on the government of Bahrain to investigate the alleged use of such excessive force," the OHCHR said in a statement.
Bahrain last year announced it was considering recommendations made in a report on the unrest from the Bahrain Independent Commission of Inquiry. BICI, in its report, concluded state security forces used unnecessary lethal force during a crackdown last year.
The U.N. rights office, however, said it was mindful of a new code of conduct issued by Manama calling on police to be respectful of human rights principles.
"We welcome this new code and hope that its implementation will be carefully monitored," the United Nations said.