* Vietnam Airlines to buy 10 A350 jets for $3.1 bn
* VietJet orders 20 A321s for $2.4 bn
* Jetstar Pacific orders 10 A320 planes for about $1 bn

Vietnamese airlines plan to order 40 Airbus jets in deals worth an estimated $6.5 billion, the European planemaker said on Tuesday, as they expand their fleets for a small but fast-growing market.

Strong economic growth and a burgeoning middle class has increased demand for travel both domestically and abroad, spurring carriers to increase routes.

In deals announced at the start of a two-day visit to the Southeast Asian nation by French President Francois Hollande, Vietnam Airlines, the country's flagship carrier, reached a preliminary agreement for 10 A350 planes worth $3.1 billion.

The widebody aircraft will allow the airline to expand its long-haul network, beginning with services between Ho Chi Minh City and Los Angeles.

Budget airline Jetstar Pacific - controlled by Vietnam Airlines and 30 percent owned by Australia's Qantas Airways - finalised its order for 10 A320 single-aisle planes valued at about $1 billion.

VietJet, the country's only private airline, placed a firm order for 20 A321s - worth $2.4 billion.

VietJet has been rapidly expanding both at home and in Southeast Asia and the deal comes on top of an order for 100 Boeing 737 MAX 200 jets in May worth $11.3 billion at list prices - the biggest aircraft order in the country's history.

The CAPA Centre for Aviation said in January that VietJet commands 40 percent of Vietnam's domestic market and it will likely surpass Vietnam Airlines this year as the country's biggest domestic carrier.

(Reporting by Ho Binh Minh; Editing by Edwina Gibbs)