LONDON (AP) _ A Liberian oil tanker broke in half in stormy weather in the north Atlantic today, burst into flames and began to sink, spreading fire over the ocean and leaving the 27 crewmen missing, coast guard officials said.

A Canadian military aircraft flew over the scene nine hours after the tanker sent a distress signal, and it spotted two empty life rafts, said Paul Kendrick of the Canadian Search and Rescue Coordination Center in Halifax, Nova Scotia. ''We have not found any survivors,'' Kendrick said.

Earlier, the Soviet ship Passat rushed from 26 miles away to help the 65,700-ton Odyssey but was unable to get closer than a mile to the stricken vessel because of the fire, authorities said.

The Passat, which monitors weather on the ocean, told rescue officials it found no sign of the Greek and Honduran crew and was searching the area, Lloyd's Shipping Intelligence Unit said.

''It doesn't sound very good at all,'' said a spokesman for the British coast guard in Falmouth, England. ''The Soviet vessel has lost radio contact with the tanker.''

Kendrick said the fire was mostly out. The stern of the ship sank and only the bow was afloat, and a large oil slick coated the ocean surface, he said. He said a bulk carrier and another Canadian aircraft were en route to the scene.

Capt. Pat Phelan of the Canadian rescue center said the tanker was about 700 miles east-northeast of Newfoundland. British officials said it was about 1,200 miles west of England.

Polembros Shipping Ltd. of London, operators of the Odyssey, said half the crew was Greek, including the captain and all the officers, and the other half was Honduran.

''We haven't heard any news at all about them,'' said a spokeswoman, commenting on condition she was not identified. She said the tanker is owned by Diamond Port Shipping Corp. of Monrovia, Liberia.

Phelan said the tanker was 52 degrees 38 minutes north latitude and 34 degrees 39 minutes west longitiude, or near the center of the North Atlantic.

The Odyssey, built in 1971, sent a distress signal at 3:22 a.m. saying it needed immediate assistance, the Falmouth spokesman said.

The signal was picked up by Valentia Radio in southern Ireland and relayed to British coast guard officials, he said. The British alerted the Passat, one of several Soviet weather ships that operate in that sector of the Atlantic.

The tanker is in the Atlantic sector of responsibility covered by the Canadians, and the emergency operation was being coordinated in Halifax with assistance from British coast guard officials.

Lloyd's Shipping Intelligence Unit in Colchester said Valentia Radio reported that the Odyssey left Sullom Voe, a Scottish oil terminal, on Nov. 5 and was heading for the Canadian port of Come By Chance.

Canadian coast guard officer James Cormier said the weather at the tanker site was stormy, with 46 mph westerly winds and 25-foot waves.