The streets of the capital of the Central African Republic were deserted, with terrified residents sheltering indoors and tens of thousands fleeing their homes after three days of shooting and bloodshed.

"We fear that the violence we're seeing in Bangui is a return to the dark days of late 2013 and 2014, when thousands were killed and tens of thousands had to flee their homes," UN refugee agency spokesman Leo Dobbs told reporters Tuesday.

At least 36 people have died in the last three days and 27,400 fled their homes amid the latest flare up of violence in the conflict-torn country, the United Nations said.

Fears of a sudden refugee influx saw neighbouring Democratic Republic of Congo announce the immediate closure of its northern border with the landlocked former French colony.

One in 10 Central Africans -- 460,000 people -- have sought refuge outside the country, mainly in Cameroon, Chad, DR Congo and Congo, since violence broke out more than two years ago.

Announcing she was cutting short a visit to the UN headquarters in New York, interim president Catherine Samba Panza said on national radio: "I appeal to you my compatriots for calm. I ask you to return to your homes."

Gunfire was heard in the afternoon in the Combattant neighbourhood next to Bangui's international airport, where some 20,000 people have taken refuge near French and UN military bases.

"There is great difficulty getting to the airport. There are barricades in the streets and there was shooting going on this morning," said the UNHCR's Dobbs in Geneva.

"The displaced people are reported to be in a state of shock."

- 'Anti-balaka fighters gather' -

Residents said members of the feared "anti-balaka" (anti-machete) Christian militia, which sprung up in 2013 to defend against mainly Muslim Seleka fighters, had begun gathering in Bangui on Monday.

"Groups of them, armed with machetes, have taken up positions in the streets of the 8th and 5th districts," one of the few residents to venture out into the streets told AFP.

The fighters were positioned near the city's PK-5 shopping area, the last bastion of Muslims hounded out of other areas by the Christian militia.

The latest escalation in two years of unrest began in PK-5 when a young Muslim motorcycle-taxi driver was murdered at the weekend, angering Muslims who used grenades and guns in counter-attacks on Christians in nearby districts.

Around 100 people were wounded in the bloodshed, prompting the government to impose a curfew on the capital.

The PK-5 area was the epicentre of an unprecedented wave of violence pitting majority Christians against minority Muslims in late 2013 and early last year.

In Bangui, terrified residents fled to camps by the airport, where French and UN peacekeepers from the 10,000-strong MINUSCA force are based.

MINUSCA denied reports that its troops on Monday killed three people and injured others after opening fire on a crowd of several hundred demonstrators heading towards the presidency to demand Samba Panza's resignation.

- 500 prisoners escape -

UN spokesman Rupert Colville said in Geneva that some 500 prisoners had escaped from Bangui's main prison overnight, adding to the climate of insecurity.

"This is a huge setback for the preservation of law and order, and for the fight against impunity, which has been and remains a chronic problem in CAR," he told reporters.

Overnight, shooting erupted as security forces tried to stop looters from attacking the premises of several humanitarian organisations, which had been evacuated for security reasons, a military source told AFP.

The Central African Republic descended into bloodshed more than two years ago after longtime president Francois Bozize, a Christian, was ousted by Seleka rebels, triggering the worst crisis since independence in 1960.

Though the tit-for-tat sectarian attacks have subsided significantly since last year, Bangui is still plagued by violent crime, fuelled in part by easy access to weapons left over from the conflict.

Amnesty International on Wednesday warned illegal trading in diamonds, which are technically banned from export under a 2013 deal, could also fuel violence as militias profit from their sale.

The International Committee of the Red Cross said it was "alarmed by the deteriorating humanitarian situation in Bangui" and had not been able to work in the capital since Sunday.

The UN's humanitarian coordinator in the country, Aurelien Agbenonci, strongly condemned attacks against the aid organisations, adding: "All perpetrators of crimes against humanitarians will be held accountable."

Looters targeted the offices of the UN World Food Programme, French medical NGO Premiere Urgence and the Dutch NGO Cordaid, police said, indicating that they had repelled them in several places.

Presidential and legislative elections are due by the end of the year, but have already been pushed back several times.
Source: AFP