The Rwandan Senate on Tuesday unanimously endorsed constitutional amendments that would allow President Paul Kagame to stay in office for another two decades.
The lower house of parliament endorsed the changes earlier this month. They will now be put to a national referendum, where they are expected to win easy approval.
Kagame, 58, has ruled Rwanda since his army ended the 1994 genocide and ousted Hutu extremists from power.
Under current law, he has to step down at the end of his second elected term in 2017. But the proposed amendments would allow him to run for another seven-year term, followed by two five-year terms, potentially extending his rule until 2034.
The opposition Democratic Green Party of Rwanda said changing the constitution would be a challenge to "sustainable peace and security." It said it would run a "no change" campaign ahead of the referendum. Officials have not yet announced a date for that vote.
Rwanda is one of the United States' closest African allies, but Washington believes Kagame should not try to keep power beyond the current limit.
"We expect President Kagame to follow through on the commitments he has made previously to foster a new generation of leaders in Rwanda and to step down at the end of his current term in 2017," State Department spokesman Mark Toner said Tuesday.
Toner did not say what steps the U.S. would consider if Kagame tried to hang on to power. Efforts by leaders in other African countries to extend their rule, including Burkina Faso, Burundi and Senegal, have led to violence.
Rwanda has so far seen no protests against Kagame, who has won widespread popular support by maintaining peace and building up the Central African country's economy. But critics say his government suppresses freedom of speech and does not tolerate dissent.