Lava flows this morning reached the centre of Goma in the Democratic Republic of Congo after the eruption of Mount Nyiragongo.
It is creating a five foot wall of cooling stone burning everything in its path and has left half a million people homeless.
About 100,000 people had fled west, while 300,000 others crossed over to Rwanda in the east. The lava cut the only road connecting the two sides.
Many crossed the Rwandan border into neighbouring Gisenyi on board overcrowded ferries on Lake Kivu which has already been polluted by the sulphur from the Volcano.
Major Pierre Pinchart, the military attache at the Belgian embassy in Kigali, said the refugee problem could develop into a major humanitarian disaster, with two infants already dead from starvation and dehydration. He said refugees have begun drinking the lake water, which could create a cholera epidemic.
More than a dozen earthquakes shook the region every hour, some of them severe enough to send people running into the street in panic. Hundreds of thousands of Goma residents, ordered to abandon their city as it burned, slept outdoors in Gisenyi, Rwanda, seeking shelter under porches.
Officials were trying to determine the number of casualties amid concerns there could be many dead. Mr Stephen Johnson, an official in the UN office for the co-ordination of humanitarian affairs in New York, said there were no indications the eruptions that began on Thursday had finished.
PA