The Category 5 storm -- which could be the island's most violent on record -- is on a painstakingly slow path through the Caribbean and expected to make landfall on the Jamaican coast on Tuesday.

"1.5 million people may be impacted," said Necephor Mghendi, the IFRC's head of delegation for the English- and Dutch-speaking Caribbean.

Speaking from Trinidad and Tobago, he told reporters in Geneva that that number could be "an underestimate".

"The humanitarian threat is severe and it's immediate," he said.

Mghendi said the Jamaican Red Cross had mobilised at its highest level but "coastal communities remain financially strained, informal settlements are highly exposed to destructive winds and already saturated soils.

"This is because it's been the rainy season and there is an increased likelihood of landslides and life-threatening floods," he said.

- 'Catastrophic' flooding predicted -

The World Meteorological Organization said the hurricane was expected to have a "massive impact".

"The wind for Jamaica is expected to reach about 280 kilometres per hour, with gusts picking up to 350 km per hour," said Anne-Claire Fontan, the WMO's tropical cyclone specialist.

"For Jamaica it will be the storm of the century so far. That's for sure."

She said up to 700 millimetres of rainfall was expected.

"It means that there will be catastrophic flash floodings and numerous landslides," said Fontan.

"In addition to the rain and the destructive wind, you will have a storm surge that is expected as well on the southern coast of Jamaica, 3-4 metres high, in addition to destructive waves.

"We have to expect the worst for this situation unfolding now."

Jens Laerke, spokesman for the UN's humanitarian agency OCHA, said the first duty was "to save as many lives as possible" by limiting the impact.

"When you have massive flooding, one of the biggest problems is water," he said, warning of "all kinds of health risks and epidemic risks if you don't have clean water."

After hitting Jamaica, Melissa is forecast to cross over eastern Cuba on Tuesday night.