Lava flows from a volcano ignited a gas station in the Congolese town of Goma, killing at least 50 people. Tens of thousands of Goma residents who fled the erupting volcano late last week are returning to the devastated city.

The latest disaster occurred as people tried to siphon fuel from gas station tanks. The tanks ignited in a massive fireball, leaving a huge black cloud over Goma for several hours.

The gas station is located near one of the main lava flows that continue to surge through the city. People were collecting fuel in the hope that they could sell it to buy supplies.

Most of Goma's residents have had little to eat or drink since Mt. Nyiragongo erupted on Thursday.

Aid agencies have been working to set up refugee camps over the border in Rwanda, where about 300,000 people first fled when the volcano exploded. The camps are now ready, but they are almost empty.

The U.N. World Food Program estimates that there are 5,000 people in the camps. They say 240,000 others returned home to Goma on Sunday, despite the risk of further volcanic activity.

One of those who returned said conditions were terrible in Rwanda. He said he was sleeping out in the open without food or water, and that he would rather die in his own country.

But there is no relief for the refugees in Goma either. Nigel West, of the aid agency World Vision, described the conditions as absolutely appalling, with people sleeping on the streets in the mud and rain.

None of the international aid agencies are distributing food because they say the lava flows make it too dangerous to set up distribution centers. Instead, the agencies are trying to find safer sites outside the city.

Brenda Barton of WFP says it is impossible to help people when they are constantly on the move.

"We have a huge responsibility," she said. "Do we give assistance to people that have flooded back into an area that we know to be dangerous? Do we give assistance in that very spot or do we try and locate it in another area? And how do we help people to get organized in order to get that assistance? This is a very, very complex situation. You probably have the largest movement in the shortest period of time that this region has ever seen."

"Basically 300,000 people spilling over and then spilling back over the border within three days," added Ms. Barton. "I have not seen that in my 10 years here. If they are flooding all over the place, how can you actually expect that people are going to get assistance?"

Mr. West of World Vision says thousands have given up on Goma and are moving on to the town of Kirotshe to the northwest.

"People have come out of Rwanda, they have reached Goma but they have not wanted to stay on that side of Goma." he said. "They have crossed over this lava stream. One person estimated that 15,000 people every hour are crossing over the lava. The lava is still molten at the bottom, but there is a crust of very hot rock on top of the molten lava and people are using this as a pavement to get across to the far side."

But the World Vision official says the areas where the refugees are going are already short of food and that the refugees will not be able to find either food or shelter. He says doctors from Goma Hospital who are working in Kirotshe report that one person has already died from cholera and six others are sick.