Nigeria's government has cautioned its citizens, in particular those with medical conditions, against travelling via Ethiopia's main airport due to its altitude.
The foreign ministry in Abuja said 16 Nigerians with underlying health problems had died this year while transiting through Addis Ababa.
The city sits at an altitude of 2,334m (7,657ft) above sea level, making it one of the highest capitals in the world. Bole International Airport is a major hub for travel between Africa, the Middle East and Asia, serving millions of passengers a year.
Nigeria said those with cardiovascular and respiratory conditions were at an increased risk.
The Nigerian foreign ministry did not disclose the identities of the travellers it said had died, nor the cause of death.
It urged anyone with a health condition exacerbated by high altitudes to "exercise caution" when planning a journey via Addis Ababa and "obtain appropriate medical guidance" before doing so.
The ministry stressed that it did not want to discourage Nigerians from travelling through the Ethiopian capital.
Visiting an area at a high altitude can be dangerous for people with heart and lung diseases, as the air is thinner and so less oxygen can get into the bloodstream, experts say.
This means the heart has to work harder to get enough oxygen to different parts of the body, which can sometimes lead to heart failure among those with existing conditions, according to the American Heart Association.
The lack of oxygen at high altitudes can cause adverse effects such as dizziness, fatigue and shortness of breath among healthy people - which can cause complications for people with lung disease, studies show.
