UN experts have come under fire in Ivory Coast as they investigate claims that president Laurent Gbagbo broke an arms embargo by importing attack helicopters from Belarus, a spokesman said today.

The latest attack on the UN came amid warnings of a humanitarian disaster as the west African nation seems to be on a downward spiral towards civil war.

The charge that Gbagbo is receiving military aid from Belarusian dictator Alexander Lukashenko came from the UN's secretary-general, Ban Ki-Moon. Gbagbo's government dismissed it as a "lie" while Belarus said it was "groundless".

Ban later qualified the allegation in comments to reporters in Washington, saying the UN-appointed group of experts in Ivory Coast had "credible information that the government of Belarus may be providing attack helicopters to forces loyal to Gbagbo".

He added: "If it is confirmed, this will be a direct violation of [the] arms embargo by the security council. We are trying to confirm this."

It was when the experts tried to search an airport in the capital, Yamoussoukro, where they believed the helicopters to be, that they were shot at by Ivorian forces, according to UN mission spokesman Hamadoun Toure.

"That's what they said," he told Reuters, adding that the investigation into the helicopters was continuing. "We are trying to have several details but so far nothing."

In New York, the UN Ivory Coast sanctions committee discussed the charge that Belarus had violated the embargo. "It looks as if the sanctions committee has not yet been able to confirm the reported arms shipment," a spokesman for the German mission said. "They requested UNoci (the UN mission in Ivory Coast) and the panel of experts to continue monitoring the situation."

Ivory Coast has been under an arms embargo since the last outbreak of serious violence in 2004, when pro-Gbagbo forces bombed French peacekeepers in the rebel-held north, but analysts say it has repeatedly been violated.

Gun battles between forces loyal to Gbagbo and his rival Alassane Ouattara, almost universally recognised as the winner of last November's election, have intensified fears of a return to full-blown civil war. Cocoa prices rose to a 32-year peak on Monday as anxiety grew.

Relations between Gbagbo and the UN mission are deteriorating, with state TV regularly broadcasting allegations about their collaboration with the rebels.

Gbago's often violent young supporters responded to a call from their leader, Charles Ble Goude, at the weekend to block UN traffic. Two UN staff members were kidnapped by a mob on Monday, then released. Toure said a third had been robbed of his wallet.

Abidjan, the commercial capital, increasingly resembles a city on the brink of war as AK-47 assault rifles and heavy weapons boom daily through the district of Abobo, the economy grinds to a standstill and thousands of people abandon their homes.

But getting out is not easy. Armed men stopped some 60 families without food or water from leaving a church today, the UN said. There were long queues of government workers trying to cash pay cheques.

The UN's refugee agency also expressed alarm about the conditions facing people trying to escape days of violence in Abobo. "There are reports of many dead bodies, buses burned and shops looted, and of young militiamen attacking people inside their homes," the agency said.

Others fleeing the violence have been forced to pay people just to get out of the area, the agency said. Nearly 70,000 people have joined a mass exodus to neighbouring Liberia.

Gbagbo's government said today that it was banning residents from filling jerrycans with petrol as fears of a shortage grow. Motorists in Abidjan rushed to fill up their car's tanks, then went home to siphon the petrol into a jerrycans. Some petrol stations had already run out of supplies and closed.

Last week, when there were skirmishes in the west of the country between former rebels and pro-Gbagbo forces, was the first time the ceasefire between the two armed sides had been broken in six years. It led to a surge in the number of civilians fleeing to Liberia, putting further strain on aid agencies.

Human Rights Watch said it had documented the recruitment and deployment of Liberian mercenaries in recent weeks, and credible sources indicated that some had fought alongside Gbagbo's forces during the recent clash.

Daniel Bekele, Africa director at Human Rights Watch, said: "Côte d'Ivoire has been down this road before, and it's been marked by grave human-rights abuses and war crimes. Both Gbagbo and Ouattara need to send clear warnings to their troops to protect civilians and prisoners from harm."

A military source told Reuters that increasing numbers of soldiers in the Ivorian military were deserting, by switching their phones off and going into hiding. A few are defecting to the other side.

Several delegations of African leaders have come to Abidjan in attempts to persuade Gbagbo to leave office. Gbagbo has rejected all their proposals, including offers of amnesty and a comfortable exile abroad. Opponents are trying to strangle his government financially, and the European Union has prohibited European ships from docking in Abidjan's port.