Two dying Ebola patients were spirited out of a Congo hospital by their relatives on motor-bikes, then taken to a prayer meeting with 50 other people, potentially exposing them all to the deadly virus, a senior aid worker said on Thursday.

Both patients were vomiting and infectious and died hours after the prayer session in the river port city of Mbandaka, Dr Jean-Clement Cabrol, emergency medical coordinator for Medecins Sans Frontieres (Doctors Without Borders), said.

The Democratic Republic of Congo is racing to contain an outbreak of the disease which spreads through contact with infected bodily fluids including vomit and sweat.

'The escape was organised by the families, with six motorcycles as the patients were very ill and couldn't walk,' Cabrol told a news briefing in Geneva after returning from the affected region.

'They were taken to a prayer room with 50 people to pray. They were found at two in the morning, one of them dead and one was dying. So that's 50-60 contacts right there. The patients were in the active phase of the disease, vomiting.'

The patients got out of the isolation ward on Monday. Earlier reports did not give details of the escape or where they went afterwards. A third patient who left the ward survived.

Health officials started trying to trace the motorcycle drivers and other people who came into contact with the patients as soon as the escape was reported, Dr Peter Salama, head of emergency response at the World Health Organization (WHO), told Reuters on Thursday.

'From the moment that they escaped, the (health) ministry, WHO and partners have been following very closely every contact,' he said.

WHO's three-month budget for the crisis has been doubled to $57 million to carry out a complex operation in a remote, forested area, Salama said.

'All it takes is one sick person to travel down the Congo River and we can have outbreaks seeded in many different locations ... that can happen at any moment, it's very hard to predict,' he said, referring to the river linking the trading hub of Mbandaka to the capital Kinshasa, whose population is 10 million.

'It is going to be at least weeks and more likely months before we get this outbreak fully under control,' Salama said.

Congo's ninth recorded outbreak of the disease is thought to have killed at least 22 people so far, according to government figures released on Wednesday night – lower than the last estimate of 27 after some of those deaths turned out not to be related to Ebola.

There have been major advances in medical treatment of the virus since it ravaged West Africa in 2014-2016, including the use of an experimental vaccine to protect medical staff.

But local scepticism about the dangers and the need to isolate infected patients continue to complicate efforts to contain it.

In past outbreaks, mourning relatives have caught the haemorrhagic disease by touching the highly-contagious bodies of dead loved ones, sometimes by laying hands on them to say goodbye.