BANGKOK, May 25 (Reuters) - The United States has declined to sell its F-35 stealth fighter jets to Thailand over issues with training and technical requirements, the Southeast Asian country's air force said on Thursday.

Thailand, which was designated a Major Non-NATO Ally by the United States in 2003, had last year set aside a 13.8 billion baht ($407.68 million) budget for new jets to replace its aging, mostly U.S.-made F-5 and F-16 fighters.

It identified up to eight Lockheed Martin (LMT.N) F-35A jets as its target.

But sale of the fifth-generation fighters was subject to conditions that included time constraints, technical requirements and maintenance compatibility and the United States was therefore unable to offer the sale, air force spokesperson Air Chief Marshall Prapas Sornchaidee said in a statement.

The F-35 is one of the world's most advanced fighter aircraft and is considered a sensitive export sold only to the United States' closest allies, which in the Indo-Pacific includes Australia, Japan, South Korea and Singapore.

Thailand currently has 12 JAS-39 Gripen fighter jets made by Sweden's Saab in addition to its U.S.-made models, many of which have been in operation for decades.

Thailand's military has used U.S. technology going back to the Vietnam War era, when it hosted U.S. air force and navy personnel at its bases. Thailand has for many years hosted annual "Cobra Gold" training exercises with the United States.

Those warm ties have, however, been strained by the Thai military's coups against elected governments in 2006 and 2014, and concerns about overtures by army-backed governments towards rival power China.

Prapas said the air force would still replace its F-16 jets and the United States had offered the upgraded F-15 and F-16s models, which could be transferred faster.