United Airlines has increased its stake in Azul to 8%, citing connectivity with the Brazilian carrier for the move.
The Chicago-based carrier bought the shares, which raise its stake by 4.3 percentage points from 3.7%, from Hainan Airlines, Azul and United say in a joint statement.
Hainan maintains a 17% stake in Azul, the Brazilian carrier's website shows.
"Following our initial investment in 2015, connecting traffic between our airlines is at an all-time high, significantly benefiting our customers traveling between the US and Brazil," says Andrew Levy, chief financial officer of United. "Azul's strong network in Brazil, unique business model and exceptional customer service make this transaction a good long-term investment."
Azul and United have both expressed interest in forming a joint venture, something that's possible with the approval of US-Brazil open skies by the latter country in March.
"We had some initial discussions, now we need to accelerate that," said Abhi Shah, chief revenue officer of Azul, on a joint venture that month.
United president Scott Kirby said in 2017 that the carrier was "looking forward" to expanding its partnerships in Latin America to "give American and LATAM a run for their money". He named Azul, as well as fellow Star Alliance members Avianca and Copa Airlines, as carriers they would seek expanded relationships with.
Passengers can connect between Azul and United at Fort Lauderdale, Orlando and Sao Paulo Guarulhos airports under their current codeshare, the airlines say.
John Rodgerson, chief executive of Azul, told FlightGlobal in March that he hoped United would add new service to the airline's main hub at Campinas Viracopos International airport from Houston Intercontinental. Such a route would connect Azul's network to United's main Latin American gateway, as well as its US domestic network particularly to the west coast.
United serves Sao Paulo Guarulhos from Chicago O'Hare, Houston Intercontinental, Newark and Washington Dulles, FlightGlobal Diio schedules show. Azul serves Fort Lauderdale from Belem, Recife and Viracopos, and Orlando from Belo Horizonte, Recife and Viracopos.