The president of Burundi's five-day ultimatum for people to hand over illegal firearms or be treated as enemies of the state may spark extensive bloodshed in the coming days, the US ambassador to the United Nations said on Thursday.

"The United States is alarmed by the escalating violence in Burundi and dangerous, irresponsible rhetoric on the part of the government, loyalist militias, violent anti-government forces and criminal elements," US Ambassador Samantha Power said in a statement.

"The United States expresses its extreme concern that the five-day ultimatum issued by the president will trigger widespread violence beginning this coming weekend," she said.

President Pierre Nkurunziza on Monday set a November 7 deadline for Burundians to voluntarily give up their guns or risk being "dealt with as enemies of the nation" after months of protests over his re-election in July and a failed coup.

READ: Burundi president sets deadline to give up guns

Power said Washington was "deeply concerned" by Nkurunziza's remarks, saying he promised the security forces would use violent methods to search homes for weapons and opposition figures once the deadline expires.

She said other government officials have used "incendiary and divisive speech." Power quoted the president of Burundi's senate, Reverien Ndikuriyo, as saying: "You have to pulverise, you have to exterminate - these people are only good for dying. I give you this order, go!"

"Such dangerous speech and the president's call for a widespread, indiscriminate security crackdown exacerbate an already volatile situation and risk inciting even greater violence," Power warned.

Power demanded that the government immediately allow African Union human rights and security monitors unfettered access to the country and to act on its commitment to take part in internationally mediated dialogue to bring stability to the country.

Burundi, which emerged from civil war a decade ago, was thrown into turmoil over Nkurunziza's plan to seek a third term in office, which many of his critics said was unconstitutional.

Nkurunziza ultimately won his bid for a new term in a disputed vote, plunged the country into crisis, including violent clashes between protesters and security forces in the capital Bujumbura and a series of targeted killings.

Experts say "tit-for-tat" assassinations of government officials and members of the opposition risk driving the nation back into another conflict and could reopen old ethnic wounds. Burundi lies next to Rwanda, where a genocide of Tutsis and moderate Hutus took place in 1994.