Nairobi (AFP) - Burundian President Pierre Nkurunziza warned rebels Thursday that they would be crushed by God after being sworn in for a controversial third term following weeks of protests and a failed coup against him.

Nkurunziza thanked God for his win in elections last month -- polls the United Nations say were not free or fair -- after taking the oath of office in a surprise ceremony in the capital Bujumbura announced only hours before.

"The victory we have achieved is a victory of all Burundians, those who elected us, and those who did not," Nkurunziza said.

His third term has been condemned as unconstitutional by the opposition and provoked months of protests. There has been a string of killings since his reelection, including of his top security chief, assassinated in a rocket attack last month.

But Nkurunziza, an ex-rebel turned born-again Christian who believes he is in power by divine choice, warned those who have chosen "the path that leads nowhere, who attack and fight their country" will be stopped by the hand of God.

"They will be scattered like flour thrown into the air -- as the God of heaven is a witness, the Burundians will be at peace," he said.

Constitutional changes?

No foreign head of state was present, but several African countries, as well as China and Russia, sent their ambassadors. European Union nations and the United States sent lower ranking officials.

In his oath, Nkurunziza swore loyalty to the constitution and "to dedicate all my forces to the defence of the best interests of the nation, to assure national unity and the cohesion of the Burundian people, social peace and justice."

But Nkurunziza also said he would review possible changes to a key clause in the constitution -- which requires ministers to come from a party with at least five percent of the national vote -- to allow the formation of a unity government.

Amid opposition boycotts, some lawmakers are technically on independent lists, making them ineligible to join the government.

But the same constitutional article also enshrines a fundamental block of the deal that ended the 1993-2006 civil war, the strict ethnic quotas in power between the majority Hutu and minority Tutsi.

Nkurunziza won over 69 percent of the vote in the disputed presidential poll last month, giving him a landslide first round victory.

But the United Nations observer mission said the vote last month was not "inclusive, free and credible" and was held "in an environment of profound mistrust" between political rivals.

Refugees urged to return

Over 180,000 Burundians have fled the country fearing further violence, according to the UN refugee agency, but Nkurunziza called for them to return.

"Those who have fled the country have done so because of rumours," he said. "We call upon all Burundian refugees to return to their homeland to participate in building their country."

Burundi's constitution only allows a president to be elected twice -- for a total of 10 years in power -- but before these polls Nkurunziza argued he had only been directly elected by the people once.

In power since 2005, when he was selected by parliament, he was re-elected in 2010.

Nkurunziza, a 51-year-old former sports teacher, was a Hutu rebel leader during the central African country's 13-year civil war, when at least 300,000 people were killed.

The opposition and international community claimed a third term violated the Arusha accords that paved the way to end the war in 2006.

Top international envoys from the UN African Union, EU and US have called on all sides to "recommit to a transparent, inclusive, and comprehensive political dialogue".

AU chief Nkosazana Dlamini-Zuma on Sunday called for "utmost restraint" by all sides, warning of potential "catastrophic consequences" for troubled Burundi and the wider region if rivals do not resolve political differences peacefully.

The army has been hugely divided by the crisis, with the leaders of the failed July coup having gone into hiding.

Many people are worried at rising tensions in Bujumbura, where gunfire has been regularly heard at night.