Most of the emerging infectious diseases China detected in recent years were imported from Africa and South America, like the Zika cases in previous months, Li said.

But "China has just one direct flight with those areas and most passengers arrive after multiple transfers," he added. "These pose a great challenge for the country's entry checks and quarantine."

In response, the General Administration of Quality Supervision, Inspection and Quarantine has strengthened quarantine and checks.

All four flights bound for Beijing from Angola have been subject to onboard inspection to prevent the spread of yellow fever, the Beijing Entry-Exit Inspection and Quarantine Bureau said on Tuesday.

Inspection and quarantine officers will inspect all passengers on these flights before they are released from the planes and check whether mosquitoes are brought in with baggage and containers, the bureau said.

In particular, for passengers on the only direct flight from Angola to Beijing, officers will check passengers' certificates for yellow fever vaccination, the bureau said.

Those without a certificate will be quarantined for six days, the bureau said. Officers will also intensify inspection of those bound for Angola and other countries with a yellow fever epidemic, said Wu Riga, an official at the Capital Airport Entry-Exit Inspection and Quarantine Bureau.

"We will inquire whether these passengers have received vaccinations and if not, we will suggest they take precautions and receive vaccinations in Angola or take measures to prevent being bitten by mosquitoes," she said.

Wu also urged all passengers from Angola and other countries with the epidemic bound for China to declare to airport staff if they have symptoms such as fever.