The U.S. State Department on Tuesday denounced recent political violence in Burundi ahead of the country’s May 17 referendum to amend its constitution and expressed concerns that the vote to extend its president’s term in office could hurt its democratic institutions.

“We denounce the numerous instances of violence, intimidation, and harassment committed against perceived opponents of the referendum,” it said in a statement. “We call on the government to respect Burundi’s international legal obligations regarding the rights to freedom of expression, peaceful assembly, and association.”

Burundi courts on Monday sentenced an official in the country’s ruling party to three years in jail for pushing to have opponents of the referendum thrown into a lake. Nearly 430,000 people, including opposition politicians, have fled the East African nation of 10.5 million people since President Pierre Nkurunziza won a third term in a 2015 election that led to violent clashes.