Hundreds of villagers have fled their homes on the slopes of Mount Ol Doinyo Lengai, in northern Tanzania, fearing an imminent volcanic eruption.
A BBC correspondent says lava is flowing from one side of the mountain, causing panic among the villagers.
Tremors measuring six on the Richter scale have shaken the area over recent days with aftershocks reaching Kenya.
Some 3,000 villagers were displaced last year after a volcanic eruption from the mountain.
Several mud-walled houses belonging to the Maasai herdsmen collapsed following tremors triggered by volcanic activity.
'Surprised'
The BBC's Josphat Makori, at the slopes of Mount Ol Doinyo Lengai, says many villagers have been spending nights in the cold and are terrified.
"They have started moving their animals in fear that the situation may get worse," our correspondent says.
The local district commissioner, Jolka Wilson Kasunga, has told the BBC News website that plans are under way to evacuate some 5,000 villagers.
"Ol Doinyo Lengai is a live, volcanic mountain. It erupts almost annually but we are surprised that this time it has been accompanied by an earthquake and this is worrying us," Mr Kasunga said.
East Africa's Great Rift Valley runs along a geological fault-line, but has largely escaped major quakes in recent years.
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