The United States has said Burundi's elections Tuesday lack credibility and by pressing ahead, the government "risks its legitimacy."

The voting "will further discredit the government," State Department spokesman John Kirby said, after polling went ahead despite opposition protests that President Pierre Nkurunziza's candidacy was unconstitutional.

The voting took place amid gunfire and grenade blasts, and at least two people were reported killed.

Opposition and civil society groups have denounced Nkurunziza's candidacy as a breach of the constitution and of a 2006 peace deal that ended a dozen years of civil war.

"The government's insistence on going forward with presidential elections on Tuesday risks its legitimacy in the eyes of its citizens and of the international community," Kirby said.

"More serious is the risk of unravelling the fragile progress made through the implementation of the Arusha Agreement, which states clearly that no Burundi president shall serve more than two terms in office," he added.