SINGAPORE - Singapore Airlines (SIA) has signed a letter of intent to buy 39 Boeing aircraft - 20 B777-9s and 19 B787s - for additional growth and fleet modernisation through the next decade.

The deal inked on Thursday also includes options for 12 more planes.

The aircraft will be delivered after 2020, SIA said in an announcement on Thursday.

The proposed order, which is valued at US$13.8 billion ($19.5 billion) based on published list prices, includes flexibility for the SIA Group to substitute the B787-10 orders for other variants of the 787 family.

"Today's major order for widebody aircraft enables us to continue operating a modern and fuel-efficient fleet, providing the SIA Group with additional expansion opportunities to ensure that we retain our industry-leading position," said SIA's chief executive officer Goh Choon Phong.

"We are continuing to invest for the future of the SIA Group. This order is also another demonstration of our commitment to further growing the Singapore hub, as we will be able to offer even more travel options for our customers," he added.

SIA is a longstanding Boeing customer with more than 50 current-generation B777 aircraft in service. Subsidiaries SilkAir, Scoot and SIA Cargo are also operators of Boeing aircraft.

In addition to the 30 previously-ordered B787-10s, SIA has outstanding orders with Airbus for five A380-800s and 57 A350-900s.

SilkAir has outstanding orders with Boeing for 37 B737 MAX 8s, while Scoot has orders with Boeing for eight B787s.

Tigerair has orders with Airbus for 39 A320neos.