The threat of a volcanic eruption has triggered evacuation orders on the eastern Caribbean island of Saint Vincent and the deployment of several cruise ships to house thousands forced to leave their homes.

Prime Minister Ralph Gonsalves said the island was on red alert after a shift in volcanic activity at the La Soufriere crater and told residents in northern areas to move to safety immediately.

"I have issued an evacuation order to all residents living in the RED ZONES on the North East and the North West of the island," he wrote in a tweet late Thursday.

The order came after scientists monitoring the volcano warned authorities they had detected magma moving towards its surface.

The island's National Emergency Management Organisation said there was a "substantial prospect of disaster."

The volcano was spitting steam late Thursday, according to images shared by the University of West Indies Seismic Research Centre, which monitors volcanic activity in the eastern Caribbean.

Around 16,000 people live in the declared red zones on what is the biggest island in the Saint Vincent and the Grenadines chain.

Royal Caribbean International and Celebrity Cruises said in a press release they were sending two ships to assist the evacuation effort.

Gonsalves said two more ships from cruise liner company Carnival were also on their way.

Those evacuated would be taken to shelters elsewhere in the island chain or other Caribbean territories that have offered assistance, such as Barbados and Saint Lucia, according to local media.

La Soufriere has not erupted since 1979, and its largest eruption happened over a century ago, killing more than 1,000 people in 1902.