Nigeria's government has warned its citizens, particularly those living with medical conditions, to be careful when traveling through Ethiopia's main airport because of the city's high altitude.
In a statement, the foreign ministry in Abuja said 16 Nigerians with underlying health problems had died this year while transiting through Addis Ababa. The ministry did not reveal the identities of those who died or state the cause of their deaths.
Addis Ababa lies 2,334m (7,657ft) above sea level, placing it among the highest capital cities in the world. Its Bole International Airport serves as a major hub linking Africa with the Middle East and Asia, and handles millions of passengers each year.
According to the ministry, travelers with cardiovascular and respiratory conditions face an increased risk when passing through the city.
It urged anyone with a health condition worsened by high altitudes to "exercise caution" when planning a journey via Addis Ababa and to "obtain appropriate medical guidance" before traveling.
At the same time, the ministry stressed that it was not seeking to discourage Nigerians from traveling through the Ethiopian capital altogether.
Experts say visiting high-altitude areas can be dangerous for people with heart and lung diseases, because the air is thinner and less oxygen enters the bloodstream.
That forces the heart to work harder to supply enough oxygen to the rest of the body, which can sometimes result in heart failure among people with existing conditions, according to the American Heart Association.
Studies show that reduced oxygen at high altitudes can also trigger dizziness, fatigue and shortness of breath even in healthy people, effects that can lead to complications for those with lung disease.
