Associated Press A fireball and black smoke are seen in downtown Goma, Congo, yesterday, after lava flows from the volcanic eruption ignited a gas station, killing about 50 people who were trying to siphon fuel from the tanks, witnesses said.

GOMA, Congo - Lava touched off a massive explosion and a series of fireballs at a gas station yesterday, killing at least 30 people scavenging for fuel. Still, residents picked their way across hardening slabs of lava, returning home to this town demolished by a volcanic eruption.

With most of the tens of thousands who fled last week's eruption already returned, a volcano expert declared the area reasonably safe despite continuing earth tremors. He said there were no indications Mount Nyiragongo would erupt again soon and that all lava flows had halted.

Residents scoured cooling tongues of lava for scorched sheets of corrugated iron to use as roofs for makeshift dwellings. Lava destroyed about 40 percent of the city at the head of Lake Kivu, but yesterday, the streets once again teemed with people, and many shops were open. U.N. officials said they were concerned about water and air quality.

The blast at the gas station yesterday morning showed the dangers as residents tried to move in. Lava apparently ignited fumes of gasoline and diesel fuel at the station, detonating a series of massive fireballs that sent a cloud of jet-black smoke into the sky.

People were trying to scoop fuel into plastic containers from the already burned-out station when the first explosion hit. One witness said 50 people were killed, but soldiers from the rebel organization that controls Goma said 30 died, some of them women and children.

More than a dozen 50-gallon barrels in the station storeroom exploded for hours, sending 100-foot flames skyward and engulfing the building. No bodies were visible outside.

There have been unconfirmed reports that as many as 40 people were killed in the initial eruption on Thursday, when Mount Nyiragongo - 30 miles to the north - sent out streams of lava that destroyed some 14 villages and ran through the center of Goma.

Jacques Durieux, a vulcanologist at the French Group for the Study of Active Volcanoes, said the worst of the eruption appeared to be over. He said continuing earthquakes caused by the settling of the area were the only threat.

"The active phase of the volcanic eruption is finished," said Durieux, contracted by the United Nations to assess the situation. He said most of the buildings were simple structures that can resist tremors.

In New York, Stephen Johnson of the U.N. Office for the Coordination of Humanitarian Affairs said officials were trying to determine whether the air and water were safe for Goma's 500,000 residents.

"While the population have voted with their feet and have moved back for the large part from Rwanda into Goma ... we are rather concerned about air quality and water quality," he said.

Johnson said the United Nations expects to appeal today for $15 million in immediate assistance including food and other items, coordination efforts and future assessments.

Laura Melo, spokeswoman for the U.N. World Food Program, said no more than 40,000 of the 300,000 people who fled eastward to neighboring Rwanda in the wake of the eruption last week remained there. Others had fled west, deeper into Congo.

In Gisenyi, nearby in Rwanda, U.N. emergency relief official Ross Mountain said he would consult with Congolese and Rwandan authorities on how to provide aid to the 10,000 families whose homes were destroyed by lava. He said U.N. officials were reluctant to begin a major aid operation in Goma until the city was considered safe.

As residents in Goma checked their homes and searched for materials to rebuild, Rwandan-backed rebel soldiers who control the area fired shots to warn looters.

Patrick Mazimhaka, Rwanda's envoy to peace talks in Congo's civil war, said Rwanda had sent another army battalion into Goma to help rebels secure the city, adding to the 7,000 Rwandan troops already in Congo.

"We owe Congolese people support to get through this," he said.

Joseph Conrad Kilamenge, a 33-year old shopkeeper, said his house was surrounded by the lava flows and he had initially fled west, deeper into Congo. He said the people of Goma were ready to rebuild.

"We are strong enough, we just need a little help," he said, adding that fresh water was needed and saying a camp for people left homeless should be set up in Congo.

Working with the rebels, the International Rescue Committee began distributing food and fresh water to Goma residents yesterday.

Francois Goemans, a spokesman for the European Union relief agency ECHO, said electricity was back on in parts of the city.

The Red Cross delivered chlorine to a water treatment plant, and Goemans said the water was free from harmful bacteria but mineral tests were still being run.

Dieudonne Wafula, a Congolese vulcanologist who has studied Nyiragongo for 15 years, said the volcano 12 miles north of Goma should not erupt for a few years, but that tremors could intensify in the coming days.

The 11,381-foot Nyiragongo and 10,022-foot Nyamulagira volcanoes north of Goma are the only two active ones in the eight-volcano Virunga chain. Nyiragongo last erupted seriously in 1977.