MOSCOW (AP) _ The Soviet Union's third major earthquake in eight weeks struck a mountainous region before dawn Saturday, leveling villages and killing at least six people, including five children.

The quake, measuring 6.3 on the Richter scale, struck in the southern republic of Georgia, near an area ravaged by a temblor April 29 that left more than 100 people dead.

Saturday's quake was an aftershock of the April 29 quake, according to the Earthquake Physics Institute in Moscow. The earlier quake measured 7 on the Richter scale and left 100,000 people homeless. It was followed within weeks by several aftershocks that killed more people.

Saturday's quake centered near the Georgian city of Bakuriani, a Caucasus Mountain ski resort town about 56 miles northwest of the capital, Tbilisi, said institute spokeswoman Olga Krylevskaya.

The worst damage was reported in the north-central regions of Dzhava and Tskhinval, where eight villages were almost completely destroyed and a town was leveled, according to the state news agency Tass and an independent agency, Interfax.

Five children were killed just outside the city of Gori, about 22 miles northwest of Tbilisi, according to the Georgian government. The exact circumstances of the deaths were not immediately known.

A sixth person also was killed, said the Georgian news agency Sakinform and a local journalist. They provided no details. Tass reported a seventh person was killed, but the report was not confirmed by journalists in the area.

More than 50 people were injured and taken to the Tskhinval regional hospital, Interfax quoted the Georgian Parliament press center as saying.

Georgia's prime minister, Tengiz Sigua, planned to tour the scene by helicopter Saturday afternoon, a local journalist said.

Phone service was disrupted to the isolated Caucasus Mountain region, which is located near the scene of recent heavy ethnic fighting.

Soviet Interior Ministry units helped transport victims and equipment on Saturday, and were flying over the area to assess the scale of the disaster, Tass reported.

Tass quoted government officials as saying a second smaller temblor struck at about 9 a.m., but there were no reports of additional damage.

The April 29 quake was felt in Tbilisi and as far as the Black Sea resort town of Sochi, 186 miles to the west. Tremors also were felt in the neighboring republic of Azerbaijan. No damage was reported in those areas.

Dzhava, a town of 11,000 people located about 60 miles northwest of Tbilisi, was completely destroyed by Saturday's quake. It had been heavily damaged by the April 29 temblor.

Another earthquake struck April 19 in the Soviet Central Asian republic of Tadzhikistan, killing one person, setting off avalanches and destroying hundreds of homes.

An estimated 25,000 people were killed in 1988 in the nearby republic of Armenia. Most people died when their flimsy high-rise apartment buildings collapsed. By contrast, most buildings in the Georgian countryside are no more than three or four stories.

Saturday's quake hit two days after a newspaper reported that Soviet geophysicists have found a connection between some earthquakes in the area and underground nuclear weapons tests conducted by the Kremlin.

Komsomolskaya Pravda said Thursday the findings were made by the Geophysics Institute at the Soviet Academy Sciences and reported recently to an international conference of seismologists. It did not identify the conference.

The scientists, who have worked for years with a special military research unit, found that 11 earthquakes in the southern Soviet Union between 1976 and 1984 had been ''artificially triggered'' by the blasts.