La Soufriere volcano on the Caribbean island of St Vincent has erupted in another huge explosion, sending sulphuric gas and ash over a wide area and prompting concerns about the safety of residents who have not evacuated.

The latest eruption on Monday sent fast-moving flows of hot gas and volcanic material down the south and southwest flanks of the volcano, which first erupted on Friday.

The government had ordered about 16,000 people who live in communities close to the volcano to evacuate on April 9, after the island was put on red alert due to a shift in volcanic activity at the crater of La Soufriere.

“It’s destroying everything in its path,” Erouscilla Joseph, the director of the University of the West Indies’ Seismic Research Center, told the Associated Press news agency.

“Anybody who would have not heeded the evacuation, they need to get out immediately,” Joseph said.

As yet, there have been no reports of deaths or injuries but drinking water supplies on the island have been polluted.

“There was a very large explosion – the largest to date from this event – this morning at about 4:15am [08:15 GMT],” Colvin Harry, a programme manager at NBC Radio in Kingstown, the capital city of St Vincent and the Grenadines, said on Monday.

“There has been a continuous process now of venting from the location of the volcano,” Harry told Al Jazeera.

The United Nations said the entire population of the main island is without electricity or clean drinking water because of the eruption and the ash from the volcano.

Spokesman Stephane Dujarric said about 20,000 people were in need for shelter according to reports received from the UN Office for the Coordination of Humanitarian Affairs (OCHA).

“The eruption has affected most livelihoods in the northern part of the island, including banana farming, with ash and lava flows hampering the movement of people and goods,” Dujarric told a Monday briefing.

La Soufriere last erupted in 1979, while a previous eruption in 1902 killed about 1,600 people.

The volcano started showing signs of activity in December and began erupting again on April 9. OCHA said intervals between tremors were currently between 1.5 and three hours, and warned explosions and ashfall of a “similar or larger magnitude” were likely to continue for the next few days.

Preliminary reports indicate flows from the volcano have destroyed nearby farms and structures.

“Scientists have advised this may go on for days. It may go on for weeks. It may go on well beyond that and there may be very large explosions from time to time throwing even more ash into the atmosphere,” Harry said.

“It’s not going to be a pretty picture when we get full view of things,” he said.

Experts have raised concerns about residents who refuse to evacuate.

It is unclear how many people have remained in their homes but one government minister who toured the island’s northeast region on Sunday said he saw a few dozen people in the community of Sandy Bay alone.

Prime Minister Ralph Gonsalves has urged people to leave, telling them it is unsafe. He has also requested assistance from the UN, which has positioned water and hygiene supplies in nearby Barbados in collaboration with the Caribbean Disaster Management Agency.

Gonsalves also said during the weekend that it could take four months before life goes back to normal on St Vincent.

Richard Robertson, a seismic researcher, told NBC Radio that the volcano’s pyroclastic flows – which the National Geographic TV network defines as “dense, fast-moving flow[s] of solidified lava pieces, volcanic ash, and hot gases” – would have razed everything in their way.

“Anything that was there – man, animal, anything … they are gone,” Robertson said. “And it’s a terrible thing to say it.”