Ethiopian authorities have detained several foreign pilots, including Briton Maurice Kirk, who were participating in an air rally across Africa, according to the United States embassy.
The US embassy said officials are working with Ethiopian authorities to secure consular access to Americans who are among the pilots detained.
The Vintage Air Rally billed itself as a “flying rally across Africa, from Crete to Cape Town”, for planes built in the early 20th century.
The head of the Ethiopian Civil Aviation Authority said the pilots entered the country’s airspace illegally on Tuesday and are under investigation.
“This group entered into Ethiopia without getting permission to access our airspace,” said Wossenyelah Hunegnawm.
He said the pilots are in Gambella in western Ethiopia, where they landed their aircraft during their journey as part of the air event.
Earlier this week Mr Kirk had been reported missing but was found safe and well.
It is unlikely the rally will proceed to Nairobi at the weekend because the crews are still held up in Ethiopia, said Tom Burns, a publicist for the air event.
In the rally, teams from around the world are attempting to cross 10 countries in 37 stops and 35 days.