Mount Soufriere volcano on the Caribbean island of St. Vincent erupted for the fifth time in two days shortly after noon yesterday, sending at least two streams of lava down the volcanic slope and filling the air with sulfur fumes and ash.

At least 20,000 people have been evacuated from the northern part of the island where the volcano is located.

Although air and sea rescue operations continue, local authorities now consider the northern third of the 198-square-mile island "effectively cut off," according to U.S. Agency for International Development disaster relief specialists monitoring the situation here.

The fifth explosion, AID science adviser Paul Krumpe said, was the "loudest yet," but there was no indication that the volcano has "climaxed."

A "climactic" eruption is considered the strongest in a series, after which there may be lesser eruptions before a volcano finally subsides.

Wire agencies reported that the eruptions had hurled a plume of ashes and smoke 20,000 feet into the air, and that volcanic dust had showered as far as Barbados, 100 miles aways. Two children were reported killed, and three other fatalities were unconfirmed.

In addition to sulfur fumes and falling debris, Krumpe said a greater danger would result if the eruptions were to force a large in the crater, containing 75 million cubic feet of water, to the rim of the volcano, creating massive mud flows.

While 4,000-foot Mount Soufriere, is considered an active volcano, it has been largely quiet since lesser eruptions in 1971. At least 2,000 people were killed in a 1902 eruption.

The first in the current series of eruptions came Friday morning, and was quickly followed by two others. During the second explosion, seismographic and other monitoring equipment placed near the crater following the 1971 eruption was destroyed.

A fourth explosion came Friday night. Two U.S. vulcanologists, part of a disaster relief effort that includes supplies flown from a Panama Canal Zone air base, three Coast Guard cutters and two rescue helicopters, arrived in St. Vincent yesterday evening.

The specialists will attempt to set up new equipment neat the crater and to analyze water, rock and ash coming from the volcano to determine the possible of larger explosions.

AID scientist Krumpe said yesterday, however, that the current series of eruptions is different, and potentially far more serious, than that of 1971, in which there was lava flow but no serious eruptions.

News services reported that evacuees were being housed in 20 to 30 relief centers, and that the government saw no need to evacuate the entire island at this time. Hundreds of persons reportedly stayed behind with their possessions.

The news agencies said that all island doctors, nurses and police vehicles had been mobilized and that the government had designated pollice stations and other public buildings as gathering points in the event of a decision to evacuate all the island's 100,000 residents. CAPTION: Picture, Mount Soufriere volcano, on St. Vincent, spews steam, sulphur fumes, ash and rock 20,000 feet into the air.AP