81 Die as Missile Downs 2nd Airliner in Georgian War : Europe: Conflict with rebels takes a vicious turn. Former Soviet republic is threatened with disintegration.

A missile apparently fired by Russian-backed separatists Wednesday struck down the second passenger plane in two days over Georgia’s breakaway province of Abkhazia, and authorities there said at least 81 people died as the aircraft burst into flames on an airport runway.

The destruction of the two civilian aircraft with heat-seeking missiles marked a vicious turn in a 13-month-old war that had appeared settled until a week ago but now threatens the multiethnic former Soviet republic with disintegration.

Since the collapse of a 7-week-old cease-fire last Thursday, Georgia’s president, Eduard A. Shevardnadze, 65, the former Soviet foreign minister who shaped policies that helped end the Cold War, has been holed up in a bunker in the obscure Black Sea resort town of Sukhumi, personally leading his army’s defense of its last stronghold in Abkhazia.

Georgian authorities accused the separatists of firing both missiles from small boats on the Black Sea at planes headed for landing in Sukhumi.

Shevardnadze’s press service said the plane downed Wednesday was a TU-154 carrying at least 100 passengers, a crew of six and a cargo of humanitarian aid from Tbilisi, the Georgian capital. After the plane was hit, it caught fire, the report said, but its pilot managed to bring it to a landing at Sukhumi’s airport. However, it exploded almost immediately on touching the runway.

Five crew members and 20 passengers escaped from the flaming wreckage, the report said.

It was the highest single death toll in a war that has claimed several thousand lives.

The TU-134 plane shot down Tuesday came from the Russian city of Sochi and carried 28 people to their deaths in the Black Sea.

The two aircraft were the first commercial planes downed in the fighting.

Abkhazian separatist leaders denied shooting down the first of the two ill-fated planes. There was no report from their headquarters Wednesday night on the second one.

But there is no doubt that the outnumbered Abkhazians have received sophisticated weapons, including heat-seeking missiles, from Russia to battle Georgia’s disorganized army.

The Russian army is formally neutral in the conflict but intent on keeping its Black Sea bases in Abkhazia, which the Georgian government wants to take over.

The war is one of a dozen ethnic conflicts reawakened by the breakup of the Soviet Union two years ago. It began in August, 1992, when the Georgian army seized the regional Parliament and the city of Sukhumi to crush an autonomy movement by 97,000 ethnic Abkhazians, who speak their own language and are a minority in their own province of 500,000 people.

Feeling oppressed by the Georgians, Abkhazians want their coastal province, a land of tangerine groves and sturdy, long-living people, to become a Russian protectorate. So do separatists in the Georgian region of South Ossetia, which was at war with the Georgian army until a 1991 Russian-brokered cease-fire that is still fragile.

There has been a struggle over policy. Russia’s diplomats are eager to make peace along the southern border, and its defense officials want to maintain Moscow’s dominance.

The Russians arranged an Abkhazian truce in July, and U.N. peacekeeping forces arrived this month to monitor it. But as soon as that conflict wound down, Shevardnadze came under attack on another front, from ethnic Mengrelian warriors led by Zviad Gamsakhurdia, the president overthrown by the Georgian army in late 1991.

By threatening to resign the presidency last week, Shevardnadze got Parliament to give him emergency powers to fight the Mengrelian insurgency. It was then that the Abkhazians launched an attack on Sukhumi, striking after the Georgians had withdrawn most of their artillery from the town under the cease-fire accord.

Russia’s Foreign Ministry condemned the invasion, and Moscow cut off electricity to the province to try to force the Abkhazians to pull back. Then the Russian Defense Ministry took the Abkhazians’ side, demanding that the Georgians complete their military withdrawal from the city.

From his bunker in Sukhumi, a city shrunken to less than half its 130,000 prewar population, Shevardnadze has appealed to world leaders for help, claiming that the loss of the city would lead to the collapse of his country.

His forces reported some progress Wednesday in pushing north from Tbilisi along the coastal highway linking Sukhumi with the rest of Georgia.

“The situation in Sukhumi is improving . . . our forces are advancing,” his chief of staff quoted the Georgian leader as saying.

But Abkhazian forces reportedly held a strong position near the airport where the two planes were shot down. The airport is a civilian terminal as well as a staging area for Georgian troop reinforcements.