Burundi Vice-President Gervais Rufyikiri flees

Gervais Rufyikiri is believed to be in Belgium

One of Burundi's vice-presidents has fled the country, saying he felt threatened after opposing President Pierre Nkurunziza's third-term bid.

Burundi has seen months of turmoil since Mr Nkurunziza announced that he would be running for office again in July's election.

Gervais Rufyikiri told France24 TV that a third term for the president would be unconstitutional.

A government spokesman denied that Mr Rufyikiri had been threatened.

There are also reports that Burundi's National Assembly Speaker Pie Ntavyohanyuma has left for Belgium, citing medical reasons. He had also criticised the president's bid for a third term.

Both men hold Belgian nationality.

Violent protests in the capital Bujumbura followed the 25 April announcement that Mr Nkurunziza would seek re-election.

The president's third-term bid has led to weeks of protest in Burundi's capital, Bujumbura

Rights groups say at least 70 people have been killed and 500 wounded as demonstrators clashed with police over the past two months.

More than 100,000 people have fled Burundi since the start of the crisis, the UN says.

In a sign of continuing tension, four people have been injured in a grenade attack in Bujumbura.

Also, more than 100 students have climbed into the compound of the US embassy to escape from the police, who were threatening to dismantle a camp that they had set up outside the building.

The governing party believes that Mr Rufyikiri was sympathetic to those behind May's coup attempt

Analysis: Venuste Nshimiyimana, BBC Africa

The fleeing of another high-profile Burundian may look like a further blow to President Nkurunziza, but it is not clear how much impact it will have on the politics in the country, as Gervais Rufyikiri had already been sidelined by the governing CNDD-FDD party.

His open opposition to the president's third-term bid and an alleged link to May's failed coup attempt led to him being pushed out of the party's influential circles.

If Mr Rufyikiri had made the move earlier then it would have been a more powerful gesture, but just days before parliamentary elections and a fortnight before the presidential poll it seems too late to change things in Burundi.

Burundi's President Nkurunziza in profile

Mr Rufyikiri fell out of favour with the governing CNDD-FDD party in April, when he told a party conference that he could not support Mr Nkurunziza's re-election.

The vice-president is the latest in a series of high-profile Burundians to leave the country, including a constitutional court judge and a member of the electoral commission.

But the government appears unperturbed, with presidential spokesman Willy Nyamitwe tweeting "goodbye and good riddance" to the vice-president.

President Nkurunziza says his first term does not count

Mr Nkurunziza's critics say that a third term contravenes the constitution, which requires the president to step down after two terms.

But Burundi's Constitutional Court ruled that his first term does not count because he was elected by parliament and not voters.

In May, the president survived a coup attempt.

The presidential election was due in June but was put back to 15 July following pressure from regional leaders, parliamentary elections are due on Monday.

The United Nations is currently trying to broker talks between the governing party and the opposition to resolve tensions.