U.S. troop strength in Thailand near that now in South VietnamBANGKOK (AP) - Reinforcement of the U.S. Air Force in Thailand has brought troop levels here to approximately that of U.S. forces in SouthVietnam.President Nixon’s announcement that Vietnam troop strength will be reduced to 39,-000 by Sept. 1 means Thailand will have at least 10,000 more U.S. servicemen than Vietnam by that time.. The U.S. Embassy in Bangkok refuses to discuss specific numbers, but a spokesman said U.S. forces in Thailand number “approximately 48,000.“ Other suurces say the figure is more than 49,000 and growing,Field Marshal Thanom Kittikachorn, chairman of the ruling National Executive Council, said there are 49,000 U.S. military personnel in Thailand.The Air Force has reopened the big base at Takhli for use by F4 Phantomfighter-bombers.A previously unuaed American-built base at Nam Phong has been opened for use by a Marine air wing transferred from Da Nang, Vietnam.A squadron of U.S. C135 aerial tankers has. been brought to Bangkok to refuel fighterbombers operating from Korat, Udnrn, Nam Phong, Takhli, Ubol and Kanom Phanom,U.S. Air Force officers are known to have surveyed the British-built air strip at Non Han near Ubol for possible use by U.S. planes if the buildup continues. ^ Reliable sources say combat air strength in Thailand has almost doubled since March to more than 700 aircraft, including 98 B2 strategic bombers based at Utapao, 90 miles south of Bangkok on the Gulf of Siam.When the buildup started after the beginning of the North Vietnameseoffensive in South Vietnam, the authorized troop strength was 32,200. Apparently moat of the, 16,000-man Increase Involves Air Force and Marine personnel transferred from Vietnam* although planes, pilots, and technical personnel also were flown directly from the United States.Except for some 5,000 Army personnel who are involved in keeping the supply lines open to the air bases from the American-built port of Sattahip near the B52 base at Utapao, most American airmen and Marines are Involved In air operations over North Vietnam, Laos and South Vietnam.The buildup has been carried out with a secrecy that has marked U.S. air operations out of Thailand since 1964. U.S. officials refuse to discuss developments except in vague terms. Entry to air bases is prohibited,