United Airlines has more than doubled its shareholding in Brazilian carrier Azul from 3.7% to 8% for an undisclosed sum.
The two airlines currently connect via gateways in Sao Paulo, Fort Lauderdale and Orlando, Florida.
Passengers connecting in Brazil have access to the largest domestic network in the country, serving more than 100 cities.
Azul operates a fleet of 120 aircraft across a network of 206 routes.
United"™s chief financial officer Andrew Levy said: "We continue to look for new ways to provide more connectivity for our customers, and we are excited about today"™s announcement as Azul has been a great partner.
"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.
"Azul"™s strong network in Brazil, unique business model and exceptional customer service make this transaction a good long-term investment."
Azul chief executive John Rodgerson added: "Today is an important step forward in our partnership with United Airlines.
"Our customers are benefiting from having access to our collective networks that extend throughout North America with United and in Brazil with Azul…