KAMPALA, Uganda — A child in Uganda has tested positive for Ebola in the first cross-border case of the deadly virus since an outbreak started in neighboring Congo last year, Uganda’s health ministry said late Tuesday, in a blow to efforts by health workers who for months sought to prevent contamination across the heavily traveled frontier. 

The 5-year-old Congolese boy has been isolated with family members at a hospital in a western district near the Congo border, Ugandan Health Minister Jane Aceng told reporters. Two family members were being tested for Ebola after developing symptoms, with results expected on Wednesday.

The announcement puts new pressure on the World Health Organization to declare the Ebola outbreak — the second-deadliest in history — a global health emergency. The outbreak is unfolding amid unprecedented challenges of rebel attacks and community resistance in a region that had never experienced Ebola before.

In April a WHO expert committee decided that the outbreak, while of “deep concern,” was not yet a global health emergency . But international spread is one of the major criteria the United Nations agency considers before such a declaration.

It was not immediately clear when the boy entered Uganda. A WHO statement said he entered on Sunday with his family through the Bwera border post. He sought treatment at Kagando hospital and was transferred to Bwera Ebola treatment unit, WHO said.