Video | One million refugees feared from Ivory Coast chaos - UNHCR

The UNHCR says most had run away from recent violence in Abidjan. "The massive displacement in Abidjan and elsewhere is being fuelled by fears of all-out war," said UNHCR spokeswoman Melissa Fleming at the agency's headquarters in Geneva.

She pointed out that the estimate of up to a million displaced was double the figure from just a week ago. Ivory Coast's population is about 22 million.

Some 52 people have been killed in Abidjan alone this week, meaning 462 have died since the stand-off began in December, according to the UN.

So far, Gbagbo has been resisting calls for him to cede power to his rival, Alassane Ouattara - widely recognised as the winner of last year's election.

Many of those fleeing are migrants from Ivory Coast's poorer northern neighbours, who went there looking for work when it was West Africa's economic powerhouse.

Some of Gbagbo's supporters have accused the migrants and their descendents of backing Ouattara and some have been singled out for attack.

Meanwhile, a UNHCR spokesman in Abidjan has also accused mercenaries from neighbouring Liberia of taking advantage of the lawlessness to loot, rape and kill in the Guiglo region, not far from where the pro-Ouattara New Forces former rebels seized the town of Blolequin earlier this week.

"They are neither pro-Gbagbo nor pro-Ouattara, they are merely profiting from the situation," Jacques Franquin told the AFP news agency.

"Guiglo is in a lawless zone, there is no functioning police, everyone does what they want."

In the meantime, French President Nicolas Sarkozy has urged the UN to impose sanctions against Gbagbo and his allies. This follows calls for UN sanctions and tougher action to oust him by West African leaders.

Aid agencies are said to be unable to reach parts of the west where forces loyal to disputed President Laurent Gbagbo have been losing ground.