The 737 MAX 8 produced for Southwest Airlines is pictured as Boeing celebrates the 10,000th 737 to come off the production line in Renton, Washington, U.S. March 13, 2018. REUTERS/Jason Redmond

(Reuters) - U.S. carrier Southwest Airlines LUV.N has ordered 40 Boeing Co BA.N 737 MAX jets worth $4.68 billion at list price to help modernize its fleet, the airline said, making it the planemaker's largest global customer for the updated narrowbody.

Southwest converted options it held over the MAX 8 jets into firm orders, with 10 to be delivered each year from 2019 to 2022, the airline said in a statement on Thursday that announced its first-quarter earnings.

“This is first and foremost an extension of our fleet modernization strategy,” Southwest Chief Executive Officer Gary Kelly told analysts on a post-earnings conference call, according to a transcript.

“We have a very strong business case to replace older 737-700 aircraft given the superior operating economics of the MAX 8.”

Southwest is the world’s largest operator of 737 jets, with more than 700 of the airplanes. The airline has now ordered nearly 300 Boeing MAX jets, the most of any customer, a Boeing spokesman said. The MAX series is more fuel efficient than previous models.

Southwest warned on Thursday that the consequences of a mid-air engine explosion on a 737-700 last week will weigh on second-quarter bookings, as investigators probe the events that led to the first passenger death in the airline’s history.

The carrier posted a first-quarter profit of $438 million, excluding special items, up from $372 million a year earlier.