By Michelle Nichols and Louis Charbonneau
UNITED NATIONS (Reuters) - The United States threatened sanctions on Friday on anyone involved in violence in Burundi against those protesting the president's bid for a third term and expressed alarm at the spread of weapons among youth militia loyal to the president.
Demonstrators have clashed with police on the streets of the country's capital for nearly two weeks, saying President Pierre Nkurunziza's plan to run for office again violates the constitution and a peace deal that ended an ethnically charged civil war in 2005.
Nkurunziza registered on Friday to run for a third term after the constitutional court ruled he could stand.
Speaking to reporters after a closed-door United Nations Security Council meeting on Burundi, U.S. Ambassador Samantha Power said there was an "apparent lack of judicial impartiality that led to this decision."
"The United States is very carefully monitoring the situation and we are prepared to take targeted measures, including visa bans or sanctions, against those who plan or participate in widespread violence," she said.
Power said a report by a security adviser to the U.N. mission in Burundi a year ago warning of a possible outbreak of political violence and alleging a massive distribution of weapons to the youth militia "appears to be credible."
Burundi expelled the security adviser in April last year over the report.
"Now we hear that some of those weapons are being used. We hear of threats by the youth militia toward people who peacefully protest against President Nkurunziza's decision to pursue a third term. These are extremely alarming reports," Power said.
"There is no question that there are weapons in the hands of people not affiliated with the traditional security establishment," she said.
Nkurunziza's bid for a third term has plunged Burundi into its worst crisis since the war, which pitted rebels from the ethnic Hutu majority against the then Tutsi-led army and killed about 300,000 people.
Power said Nkurunziza's decision violated the peace deal that ended the civil war and limits the president to two terms.
The U.N. Security Council was briefed on Friday by the U.N. special envoy for the Great Lakes region, Said Djinnit. The 15-member body called on all parties to refrain from violence.
More than 50,000 Burundians have fled in recent weeks, the U.N. refugee agency has said.
Rwanda, with a similar ethnic mix to Burundi, has voiced concern about the current unrest. In its 1994 genocide about 800,000 Tutsis and moderate Hutus were slaughtered.