Three new cases of the often lethal Ebola virus have been confirmed in a city of more than 1 million people, the Democratic Republic of Congo's health minister says

Key points: The outbreak will test a new vaccine, with vaccinations starting early next week

The outbreak will test a new vaccine, with vaccinations starting early next week WHO has decided not to declare a global health emergency yet

WHO has decided not to declare a global health emergency yet They have said there should be no travel restrictions

The statement said the confirmed cases were in Mbandaka city, where a single case was confirmed earlier in the week.

The spread of the haemorrhagic fever in an urban area will raise alarm.

There are now 17 confirmed Ebola cases in this outbreak, including one death, plus 21 probable cases and five suspected ones.

The World Health Organisation (WHO) has decided not to declare the outbreak a global health emergency, but it called the risk of spread within Congo "very high" and warned nine neighbouring countries that the risk to them was high.

Health officials are trying to track down more than 500 people who have been in contact with those feared infected, a task that became more urgent with the spread to Mbandaka, which lies on the Congo River, a busy traffic corridor, and is an hour's flight from the capital and its 10 million citizens.

But the WHO said there should be no international travel or trade restrictions.

The outbreak is a test of a new experimental Ebola vaccine that proved effective in the West Africa outbreak a few years ago.

Vaccinations are expected to start early in the week, with more than 4,000 doses already in Congo and more on the way.

A major challenge will be keeping the vaccines cold in the vast, impoverished country where infrastructure is poor.

While Congo has contained several Ebola outbreaks in the past, all of them were based in remote rural areas.

The WHO, which was accused of bungling its response to the West Africa outbreak — the biggest Ebola outbreak in history with more than 11,000 deaths — appears to be moving swiftly to contain this latest epidemic, experts said.

There is "strong reason to believe this situation can be brought under control," said Dr. Robert Steffen, who chaired the WHO expert meeting on Friday.

But without a vigorous response, "the situation is likely to deteriorate significantly."

The virus has twice made it to Congo's capital of 10 million people, Kinshasa, in the past but was rapidly stopped.

A look at the confirmed cases of Ebola around the world. ( Reuters )

AP