Nairobi (AFP) - It started as street protests three months ago, with furious citizens demonstrating against Burundi President Pierre Nkurunziza's now successful bid for a third term in power.

But while the demonstrators who blocked city streets in daily protests are now gone, analysts warn the nature of violence has shifted -- and some fear they may have already seen the opening shots in a new civil war.

In Africa's troubled Great Lakes region, an area with a grim history of massacres and war, "a new rebellion is being born before our eyes", a Burundian analyst said.

The UN Security Council on Monday expressed "deep concern on the political and security" situation.

UN Assistant Secretary-General for Human Rights Ivan Simonovic warned about an escalation, with his spokesman saying "the risk of severe violence, of a spiralling out of control of the situation in Burundi is, indeed, very real".

Burundi had been slowly getting back on track after the 1993-2006 conflict, in which some 300,000 people were killed.

Back then, the battlefields were green hills and farmlands where rebels from the majority Hutu people clashed with an army dominated by the minority Tutsi.

Today, the violence is mainly in the capital, Bujumbura. Most nights, the city rattles with bursts of automatic gunfire and the blasts of grenades.

"The war, in reality, has already begun," said Innocent Muhozi, a key civil society leader.

At dawn, signs of the fighting become visible: the blackened remains of burnt-out vehicles and corpses lying in the streets.

- Opposition arming themselves -

"At night, neighbourhoods are barricaded with armed men patrolling," said one resident describing young men carrying AK-47 rifles in the dark.

"Peaceful demonstrators before the election were accused of being insurgents -- now they have become that," said the analyst, who declined to be identified.

Some 200,000 people have fled fearing further violence into neighbouring Tanzania, as well as to Rwanda and Democratic Republic of Congo.

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Nkurunziza's successful effort to bulldoze his way into a third term in a controversial July election fuelled protests, a sweeping crackdown and an exodus of citizens fleeing the unrest.

When his candidacy was announced in April, it was condemned as unconstitutional by the opposition and sparked months of protests.

In mid-May, generals attempted a coup, which failed, with its leader Godefroid Nyombare -- a powerful Hutu army commander -- vanishing. His supporters have since promised to topple Nkurunziza.

Now security forces are hesitant to return to neighbourhoods opposed to the government, fearing they will be attacked.

Earlier this month, insurgents used a rocket to assassinate top general Adolphe Nshimirimana, who was widely seen as the country's de-facto internal security chief.

Now people are living in fear of further attacks.

The city is rife with secrets and rumours of young men vanishing overnight, leaving to join an underground rebellion, or of desertions within the army.

Rebels are growing in strength with greater professionalism and "increased operational capacity", the analyst said.

A Western security source confirmed recent reports about apparently well-trained men infiltrating into the capital's opposition neighbourhoods.

Burundi's government has pointed the finger of blame at neighbouring Rwanda -- the base of many opposition figures who have fled into exile -- a claim Kigali has not commented on.

Nkurunziza, a 51-year-old former sports teacher, and born-again Christian, was a key Hutu rebel leader during the civil war.

But his opponents now appear to be fellow Hutus, not only the members of the Tutsi minority he battled in the 1993-2006 conflict.

Main opposition leader Agathon Rwasa -- who led a different Hutu rebel faction in the war, the National Liberation Forces -- denounced Nkurunziza's win but took his seat in parliament.

But on the ground, Rwasa's traditional supporters are on the "front line" in the hills surrounding Bujumbura, from where they are bringing in guns into the capital, the Burundi expert added.