US and UK government officials are warning travelers of the possibility of a concealed Ebola outbreak in Tanzania after the World Health Organization reported that the government there is withholding information about possible cases of the deadly virus.

On September 21, the WHO released an unusual statement outlining a series of unofficial reports from the country. The first was that a doctor who had recently traveled to Uganda had returned to Tanzania with a “suspected” case of Ebola. Testing performed by the Tanzanian National Health Laboratory reportedly indicated that the doctor was positive for the virus. She died on September 8 in Dar es Salaam, the largest city in Tanzania, after traveling extensively throughout the country. Subsequent unofficial reports to the WHO indicated that there were several other suspected cases as well as contacts in quarantine in various sites in Tanzania.

The Tanzanian government has said that there have been no cases of Ebola and that no suspected cases are “admitted anywhere” in the country. But officials there have been remarkably slow to respond to the WHO’s requests for information, have failed to provide critical details about the cases, have not offered alternative explanations for the illnesses and death, and have refused to perform confirmatory tests to ensure that the disease is not spreading, according to the WHO.

While the WHO ultimately determined that there is too little information to justify travel and trade restrictions at this point, US and UK government officials are now warning travelers of the possibility that the virus is spreading unchecked in the country.

The US Centers of Disease Control and Prevention and the State Department updated their travel advisories for Tanzania late Friday, September 27.

The CDC’s warning briefly summarizes the situation outlined by the WHO, noting that the doctor who died had a “probable” case of Ebola, and that she had traveled through several cities, including Songea, Njombe, and Mbeya.

The agency warned that “travelers should remain aware of the situation and avoid direct contact with people who are ill, when possible. They should also monitor themselves for symptoms of EVD [Ebola virus disease] (fever, severe headache, muscle pain, weakness, fatigue, diarrhea, vomiting, abdominal pain, unexplained bruising or bleeding) both during and for 3 weeks after travel.”

The State Department likewise suggests that people visiting Tanzania “exercise increased caution.” Its warning was more concise, but it still called the doctor’s death a “probable” case of Ebola.

As noted by STAT news, the UK government also updated its travel advice to warn of the possibility of Ebola in Tanzania.

The UK’s warning notes that the WHO declared the current Ebola outbreak in the Democratic Republic of the Congo a Public Health Emergency of International Concern (PHEIC) in July and that cases have spilled over to Uganda.

The outbreak, which began in August of 2018, has sickened over 3,000, killing more than 2,000 so far. It is the second largest Ebola outbreak on record, surpassed only by the 2014 West African outbreak involving more than 28,000 cases and 11,000 deaths.