Britain and the Soviet Union launched an evacuation of more than 200 foreign nationals from South Yemen yesterday, as fighting raged for the fifth day around the capital, Aden.

The British Foreign Office reported that Queen Elizabeth II's royal yacht Britannia, with the help of a Soviet merchant ship, was using launches to ferry evacuees from beaches.

French warships also were reported standing off South Yemen.

A Soviet-sponsored cease-fire between Marxist factions collapsed yesterday, and fierce fighting resumed. There were accounts of tank battles, including one near Aden's airport, amid reports from neighboring North Yemen that Army units were fighting on both sides, according to news agencies. The loyalties of the Air Force also were reported to be divided.

The fighting erupted Monday when hard-line Marxists led by former president Abdul Fattah Ismail and First Deputy Premier Ali Ahmed Nasser Antar attempted to overthrow the government of Ali Nasser Mohammed, also a Marxist.

South Yemen, the Arab world's only Marxist state, is one of the Soviet Union's closest allies in the Middle East. The Soviets maintain a large naval base on the island of Socotra, about 570 miles off Aden, and work as government advisers. Soviet efforts to evacuate some personnel were seen by some observers as an indication that the situation in South Yemen was deteriorating further after the aborted cease-fire.

In Moscow, the Soviet government urged an immediate end to the "internecine clashes," the official news agency Tass reported.

Politburo member Igor Ligachev, who ranks second in the Soviet Communist Party, met with South Yemeni Prime Minister Haidar Abudakar Attas and Foreign Minister Abdel Aziz Abdo Dali, who arrived in Moscow Thursday for an unscheduled visit.

The Tass report said that any attempts to interfere in South Yemen would be "impermissible" and emphasized the need for "restoration of the unity of the ruling Yemen Socialist Party."

In Washington, State Department spokesman Bruce Ammerman said, "In the past, South Yemen actively supported insurgent movements in Oman and North Yemen, both countries with which we have friendly relations and whose stability is important to us.

"More recently, the government of South Yemen ended military support for those movements and worked to improve and normalize South Yemen's relations with its neighbors. . . . If a new government came to power which did not share this commitment to living in peace with its neighbors, that would be of concern to us."

Negotiations went on all day yesterday among the governments of Britain, France, the Soviet Union and others, to decide first if the evacuation was necessary, then if it was feasible to use ships. It was decided to go ahead when fighting got particularly bad in areas such as Khormaksar, where the British Embassy is located, and Tawahi, where the Italian Embassy is.

"It was a question of them first being thought safer inside than outside," an official said in London.

The evacuation finally got under way yesterday afternoon from the beaches of Aden. Negotiations were led by British Foreign Secretary Geoffrey Howe, who had been in the Persian Gulf region this week on visits to Oman, Saudi Arabia and Kuwait.

According to the Foreign Office, the evacuation started yesterday afternoon, with small boats from the Britannia headed for shore to ferry people out. The operation was under the direction of Adm. John Garnier, flag officer of the royal yacht.

The British Defense Ministry said that 130 evacuees from 25 different countries, "mainly women and children," had been taken aboard the Britannia.

The refugees eventually could be transferred to another ship and returned to Britain, or perhaps taken across the Bab al Mandab Strait to Djibouti.

Two other Royal Navy ships, the guided missile destroyer Newcastle and frigate Jupiter, also were off the South Yemen coast, and the French Defense Ministry said its frigate De Grasse and support ship Jules Verne were in the area as well.

A British official said tonight that "we are taking aboard anybody who is a non-Yemeni. We are not there to pick up members of the losing side."

When it started to get dark, there were fears that not all British citizens -- estimated at 55 to 60 but possibly as many as 70 -- had been evacuated. A message was sent out to them from the Foreign Office via the British Broadcasting Corp.'s World Service.

"We have told them to rendezvous at 6 a.m. Saturday morning local time at the northeast corner of the Soviet compound, which is a convenient point on the beach," a Foreign Office official said.

"The situation is very confused. There are other ships there, and we don't know whether all the British evacuees are on the Britannia."

Soviet merchant ships equipped with small boats reportedly were helping the Britannia with the evacuation. More than 200 Europeans -- from Britain, France, the Soviet Union, Italy and West Germany -- are to be evacuated.

In Bonn, a Foreign Ministry spokesman said that the West German Embassy in Aden had been shelled and that some West Germans were feared trapped in a hotel. The French Embassy reportedly was damaged by shells, but there were no casualties reported, according to Reuter.

One British citizen, yachtsman Richard Cottier, 34, interviewed last night by BBC television, said he and his crew managed to flee to safety from their vessel, The Innocent Bystander, when it was hit, apparently in Aden harbor, by a gunboat.

"There was some machine-gun fire at the back of the boat, which completely sprayed the whole cockpit and started ripping through the awning and broke a couple of wires on the mast," Cottier said. "At that time, we also took a large shell from the gunboat to the bows. We pulled up anchor and made an emergency dash across the harbor. The gunboat nosed round the stern of the ship, took aim on the vessel and blasted it . . . out of the water, virtually."

The Britannia was on its way to New Zealand when it was diverted to stand off the coast of South Yemen, where it has been sitting for the past 48 hours. Queen Elizabeth, who is due to meet the yacht en route to a royal visit to Nepal, Australia and New Zealand, said she was "delighted" it was being used in the evacuation, Buckingham Palace said.