Outbreaks of deadly yellow fever in Angola and the Democratic Republic of Congo do not constitute a global health emergency but require stepped-up control measures and mass vaccination, the World Health Organisation (WHO) said on Thursday.

The disease, which has a high fatality rate, has already spread to Kenya and China and there is an unrelated outbreak in Uganda, generating fears of the mosquito-borne disease jumping to sprawling cities in Asia and Africa.

"This can be a devastating disease with rapid spread particularly in urban areas," Dr. Bruce Aylward, WHO executive director of outbreaks and health emergencies, said after its emergency committee on yellow fever held a first meeting.

"The big push really is around surveillance and laboratory diagnostics capacity so that if people start turning yellow and dying, you get diagnostics rapidly and vaccination," Aylward told Reuters.

The more than 2,400 suspect cases and 300 deaths in just four months in Angola "reinforced the potentially explosive nature of this disease and the risk internationally", he said.

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The panel of eight independent experts, led by Nigerian Professor Oyewale Tomori, said that urban yellow fever poses "serious national and international risks" but stopped short of declaring it a global emergency like the Zika virus or polio.

"Much concern was focused on (ways) to ensure it does not become what we do not want it to become," Tomori said.

Angola and Congo must step up surveillance to detect the virus and carry out mass immunisation, the committee said.

Luanda, Angola's capital where the outbreak began in December, is now reporting 90 percent coverage with the one lifetime dose of the vaccine, Aylward said.

The global stockpile of yellow fever vaccine should reach 7 million doses by the end of May and up to 17 million in late August, enough to combat current outbreaks but not if the virus spreads and causes "potentially explosive" outbreaks in other urban areas, Aylward said.

"We expect 7 million doses, especially with additional doses expected by August, should be sufficient. It is sufficient vaccine we believe to stop the transmission that we currently know (of)."

He added: "So the expectation is the current situation could be handled with the existing vaccine."

"The challenge would be of course if there are other outbreaks in other urban areas, if these prove to be explosive because of an inability to rapidly detect or vaccinate, that is when we could end up potentially in a situation of needing to look at dose-sparing strategies."

WHO is working with four vaccine manufacturers - Sanofi, Institut Pasteur (Dakar), Biomanguinhos (Brazil) and Chumakov Institute (Russia) - whose combined annual production capacity is 70 million to 80 million doses, Aylward told Reuters.