Updates with plan to buy 12 jets, air force chief and SAAB comments in paragraphs 1, 3 to 7

BANGKOK, June 4 (Reuters) - Thailand plans to acquire a dozen Gripen JAS 39 fighter jets built by Sweden's SAAB SAABb.ST over 10 years to replace its ageing fleet of mainly U.S. aircraft, the air force chief said on Wednesday, two years after Washington denied its request to buy F-35s.

Thailand has one of Southeast Asia's best-equipped air forces, with 11 JAS 39 Gripen jets and dozens of American F-16 and F-5 aircraft, some of which have been in operation since the late 1980s.

The budget for the first procurement phase from 2025 to 2029 is 19.5 billion baht ($596 million) for four Gripen jets, Air Chief Marshal Punpakdee Pattanakul said.

The air force, which had said last year that it wanted to buy more Swedish jets, said it would send the plan to cabinet for approval next month. If approved, the contract was expected to be signed in August.

"We welcome the Royal Thai Air Force's selection of Gripen E/F as its future fighter and look forward to the next steps in this procurement process," SAAB CEO Micael Johansson said in a statement.

"Gripen E/F is the best solution to provide Thailand with independent airpower for the future which will contribute significantly to the nation's safety and security," he said.

The Swedish company said it had not signed a contract nor received a firm order yet.

Thailand, which has a long history of military cooperation with the United States and was designated a Major Non-NATO Ally by Washington in 2023, had initially sought as many as eight Lockheed Martin LMT.N F-35 stealth jets, among the world's most advanced fighter aircraft.

But the United States in 2023 declined to sell them over what Bangkok said were issues with training and technical requirements including maintenance compatibility.

The F-35 is considered a sensitive export sold only to the United States' closest allies, which in the Indo-Pacific include Australia, Japan, South Korea and Singapore.

(Reporting by Chayut Setboonsarng, Panarat Thepgumpanat, and Panu Wongcha-um; Additional reporting by Stine Jacobsen in Copenhagen; Editing by Martin Petty and John Mair)

((chayut.setboonsarng@thomsonreuters.com ; +66854849033))