The World Health Organization said the second fatality was a female relative of the first case.

"It was difficult for the response team to trace her because she ran away from the team on the ground to a prayer meeting," Dr Gervais Folefack told the BBC in a phone interview.

"By the time the team got to her, it was already too late. She died before receiving any treatment. Both of them were buried in line with safety guidelines," he added.

On Monday health authorities said more than 100 people had been identified as possible contacts, and a vaccine was expected to be rolled out this week.

Congo has seen 13 previous Ebola outbreaks, including one that ended two years ago, which causes more than 2,000 deaths.

Samples analysed at the main laboratory in Kinshasa showed that the new outbreak is not linked to previous ones.

Investigations are ongoing to determine the source of the new outbreak.

Some 1,300 vaccines are scheduled to flown from Goma, in the eastern part of the country, to Mbandaka. The vaccination exercise will begin on Wednesday.