President Pierre Nkurunziza's bid to win a third term "seriously undermines Burundi's stability" and violates the Arusha Agreement that brought its 13-year civil war to an end, the U.S. said late Monday.

"Any violation of Arusha seriously undermines Burundi's stability," the statement issued by the U.S. embassy in Bujumbura said.

Burundi's President Pierre Nkurunziza, whose controversial bid to seek a third consecutive term has sparked weeks of civil unrest and a failed coup attempt, on Monday warned against any fresh move to try unseat him.

Addressing supporters in his hometown in the north of the country, Nkurunziza thanked those who backed him after a top general launched a failed coup while he was out of the country in May for a summit.

Elections in Burundi should be delayed by at least a month and a half and all violence must stop, East African leaders said Sunday after a summit on the crisis.

"The summit, concerned at the impasse in Burundi, strongly calls for a long postponement of the elections not less than a month and a half," the East African Community (EAC) said in a statement read out by its secretary general, Richard Sezibera, after a meeting of regional leaders in Tanzania.

Burundian President Pierre Nkurunziza's controversial bid to stand for a third term in office suffered a new blow on Saturday after it emerged that a top election official had fled the country.

Sources said the election commission's vice president, Spes Caritas Ndironkeye, jetted out of the crisis-hit central African nation late Friday, leaving behind a resignation letter and preparations for next week's parliamentary elections in disarray.

Burundi's main opposition parties said Wednesday it was now "impossible" to hold free and fair elections next week and that the result should not be recognized if they take place.

The opposition said plans for polls to go ahead despite widespread civil unrest in the capital was tantamount to an "electoral hold-up" by President Pierre Nkurunziza, whose controversial bid for a third consecutive term has plunged the central African nation into a deep political crisis.

Burundi's government on Tuesday condemned mounting diplomatic pressure over President Pierre Nkurunziza's controversial bid to stand for a third consecutive term, signaling it would not bow to international criticism.

The government has "red lines", spokesman Philippe Nzobonariba said on state radio, indicating that Nkurunziza was still unwilling to compromise even though the political crisis has already sparked weeks of civil unrest, an attempted coup, a refugee crisis and international isolation.

A demonstrator was shot dead and two other wounded in southern Burundi on Monday, a local official and witness said, in one of the first confirmed reports of civil unrest erupting outside the capital Bujumbura.

The protester was killed when local police opened fire on a group of around a hundred demonstrators in Muyange in Bururi province, around 60 kilometers (38 miles) southeast of the capital, the sources said.

On the streets of Burundi's capital, protesters have spent a month fighting running battles with police, erecting barricades and demanding President Pierre Nkurunziza end his third term bid for power.

Here in the lush green farmlands outside the capital, in the rolling hills of Bundanza district, die-hard supporters of the ruling CNDD-FDD party say the president is a prophet.

Thousands of Burundian activists mourned a murdered opposition leader at his funeral Sunday, vowing to intensify their protests against a bid by the president of the central African nation to seek a third consecutive term.

Opposition leaders also said they were suspending talks with the government of President Pierre Nkurunziza after Zedi Feruzi, the leader of the Union for Peace and Development (UPD), a small opposition party, was gunned down in the capital on Saturday.

Burundian anti-government protesters began a two-day truce Saturday after almost a month of deadly violence triggered by the president's bid for a third term culminating in a deadly grenade attack Friday.

The streets of the capital Bujumbura were calm and quiet, with some shops open and a few policemen sitting casually at junctions and roundabouts -- a marked difference to protests that have left over 20 people dead since the crisis erupted in late April, and which has included a failed coup.