Twenty-one people have died of yellow fever in Congo, some of them from infections contracted in neighbouring Angola, the World Health Organization said on Tuesday.

An outbreak has already killed 225 people and infected 1,600 in Angola. There is now a high risk of further spread in Congo, given the number of people who regularly travel between the two countries, the WHO said.

Worst outbreak in 3 decades

Just last week, the organization's director-general visited the Angolan capital of Luanda to assess the situation on the ground and called it "the most serious outbreak of yellow fever" the country has seen in 30 years.

"WHO is taking urgent action to support the government to control this outbreak with a widespread vaccination campaign," Dr. Margaret Chan said in a statement.

But a vaccine shortage is hampering efforts to keep the spread of yellow fever under control, Chan said.

By the end of March, 5.7 million people in Luanda had received yellow fever vaccinations from the International Co-ordination Group emergency stockpile. But at least another 1.5 million doses are required for that region alone, and the WHO said the stockpile is almost depleted.

Yellow fever is a transmitted by mosquitos — most commonly by the same species that spreads Zika virus.

​Most people infected with yellow fever experience fever, nausea, vomiting and fatigue. Symptoms usually improve within three to six days.

However, about 15 per cent of infected people experience a second, more severe phase of the illness, which causes high fever, jaundice and internal bleeding. The more severe cases of yellow fever are fatal about half of the time, the WHO says.