Around 10 North Korean workers dispatched to Angola have died of yellow fever in the southern African country, a U.S. radio outlet said Friday.

About 450 people have been confirmed to be infected with the viral disease in Angola since the first case was reported in December in the country's capital, the Washington-based Radio Free Asia (RFA) report said.

Among those that caught yellow fever, 178 have died so far, with the disease spreading rapidly in nearby countries, the RFA said, quoting sources.

It said some 1,000 North Korean workers are in Angola, including construction workers and medical staff, the report said, referring to the workforce North Korea dispatches overseas to earn money.

The recent deaths of the North Koreans calls into question the quality of North Korea's yellow fever vaccine and the veracity of North Korea's claims to have inoculated its workers sent to the African country, according to the report.

Those who became sick have asked to be repatriated, but the North Korean government has opted to not comply out of fear that they could cause the disease to spread at home, the media company said. 

In a separate story, the RFA also said China's customs authorities have recently tightened their control of cargo at its borders with North Korea.

Customs officials increased the frequency of their random inspections on cargo shipments to see if the claimed items match actual products being transported, the report said.

The tightening is especially aimed at stopping packages labeled as apples but actually containing Chinese rice from being exported to North Korea, the report said.

China's customs office has also required North Korea to win prior approval before shipping Chinese chemicals to the North, which could be used for the production of narcotics, the RFA said. (Yonhap)