The AP reports that a second earthquake, 6.1 on the Richter scale, hit Haiti about 35 miles north-west of Port-au-Prince at 6.03am.
The quake, say AP reporters in the capital shook buildings and sent "people running into the streets."
More as it comes in.
UPDATE (8.30am): The BBC say that some buildings that were already weakened by the initial tremor have collapsed. "Wails of terror filled the air," says their correspondent. They also describe the fresh quake as an aftershock.