Samantha Power, the United States Ambassador to the United Nations, said on Tuesday that Rwandan President Paul Kagame should set an example for his region and step down at the end of his second term in 2017.
"President Kagame has an opportunity to set an example for a region in which leaders seem too tempted to view themselves as indispensable to their own countries' trajectories," Power told reporters.
"Nobody is indispensable," she added.
"We expect President Kagame to step down at the end of his term in 2017."
Former rebel leader Kagame won the elections in 2003 and 2010 and has governed Rwanda since his rebel army ended the 1994 massacre.
He won international and domestic praise for rebuilding Rwanda after the genocide, in which 800,000 people, most of them Tutsis and moderate Hutus, were massacred.
The Rwandan Parliament passed on November a constitutional amendment that allows him to run for another two terms, which means he would extend his rule ‘legally’ until 2034.
The changes in the constitution will be voted on in a national referendum, but are expected to pass with little opposition.
He is generally seen as a guarantor of the economic stability and growth, 20 years since genocide, but some dissidents say the changes in the constitution are arranged by a government and leader of a country where freedom of speech is seriously cut.
Kagame has not said explicitly that he wants to run again but has made clear he is open to persuasion.
Power said that "We really do expect President Kagame to follow through on the commitments that he has made many times in the past to allow the next generation of leaders to come forward."
Kagame was among the latest of long serving African leaders who attempt to extend their rule in power.
Burundi, Rwanda’s neighbour, has been facing an internal violence since April, after President Pierre Nkurunziza announced he would stand again as president, despite the constitutional two-term limit, as he argues his first term does not count because he was chosen by MPs.
Samantha Power, the President of the United Nations Security Council for the month of December, also said the 15-nation council was discussing a possible visit to Burundi this month.
UN Secretary-General Ban Ki-moon has said on Monday that Burundi is on the brink of a war that could have "potentially disastrous effects in an already fragile region."
Nkurunziza was re-elected in July, which the opposition says contradicts the terms of the peace deal and the constitution signed in 2006 after ten years of civil war.