A five-year-old boy in Uganda has died of Ebola, the health minister told AFP on Wednesday, as his grandmother and younger brother also tested positive for the virus after joining the child on a visit to neighbouring Democratic Republic of Congo.

Health Minister Ruth Aceng said Uganda had now recorded three cases of Ebola.

It marks the first known cross-border spread in an epidemic that began in eastern Democratic Republic of Congo (DRC) last August. More than 2 000 cases have been recorded there, around two-thirds of them fatal.

Uganda's health ministry said on Tuesday that a woman of Congolese origin, who is married to a Ugandan, had gone to the DRC with her mother, two children and one other family member to take care of her father, who later died of Ebola.

Upon their return to Uganda, the five-year-old boy was vomiting blood and taken to hospital, and lab tests revealed he had contracted the haemorrhagic virus.

The family were quarantined, Aceng said.

Blood tests later confirmed the boy's three-year-old brother and 50-year-old grandmother also had Ebola.

"We have three cases of Ebola confirmed. Unfortunately, we lost the boy who was first tested and was found positive," Aceng said.

"We have put all those that got in contact with the boy in isolation ward for monitoring."

The child was buried late Wednesday in Kasese, in Uganda's west, health ministry spokeswoman Emma Ainebyoona told AFP.

Eight other people in contact with the family had been tracked down and were being monitored.

They, and frontline health workers, would be vaccinated Friday with a new drug designed to protect them against the virus.

While they were in the DRC, the family were identified as having been in contact with an Ebola patient and placed under quarantine, Congolese Health Minister Oly Ilunga Kalenga said.

But they left the isolation ward and crossed into Uganda, he added.

"As soon as they crossed, we contacted the Ugandan authorities," he told AFP in an interview Wednesday.

High alert

East Africa has been on high alert since the outbreak was declared in the eastern DRC provinces of North Kivu and Ituri last August.

According to the WHO, Uganda has vaccinated nearly 4 700 health workers in 165 facilities.

Uganda has experienced several outbreaks in the past, most recently in 2012, while in 2000 more than 200 people died in an outbreak in the north of the country.

South Sudan has also declared a state of alert and vaccinated health workers.

The Red Cross said it was scaling up efforts to contain the spread of the virus since it was detected in Uganda.

"This is a worrying development, but we have been preparing for this day for months now," Robert Kwesiga, Uganda Red Cross Secretary General, said in a statement Wednesday.

The United States pledged to continue providing assistance to Uganda, saying it had full confidence in the government's ability to respond to the outbreak.

The DRC has struggled to contain the virus, which has left over 1 300 dead - a mortality rate of around two-thirds.

Efforts to tackle the crisis have been hampered both by militia attacks on treatment centres and by the hostility of some local people to the medical teams.

Five workers have been killed, according to an AFP tally, and important preventative work, such as vaccination programmes and burials of Ebola victims, has been delayed.

The outbreak is the 10th in the DRC since the disease was identified in 1976.

It is the worst on record after an epidemic that struck Liberia, Guinea and Sierra Leone between 2014-2016, leaving more than 11 300 people dead.

Ebola is transmitted to people from wild animals and spreads among humans through close contact with the blood, body fluids, secretions or organs of an infected person. The virus's "reservoir", or haven in the wild, is believed to be a fruit bat native to tropical Africa.