The US has provided replacements for four F-16 fighter planes lost in crashes in 1998 and 1999, according to a defense source who requested anonymity.
With the four replacements, Taiwan once again rosters 150 F-16s. The original sale of 150 jets was approved by the US in 1992.
However, the four replacement F-16s are not new, but planes that previously saw service with the US Air National Guard, the source said.
Currently, the planes are being rostered at a flight combat center in the US state of Arizona where Taiwan's air force maintains a pool 10 F-16s for training purposes.
The replacements were delivered earlier this year, with the four former US Air National Guard F-16s sent to the Arizona air center to join 10 Taiwanese-owned of F-16s already there.
Shortly thereafter, four F-16s previously used by Taiwanese pilots for training were shipped to Taiwan. With the transfer, the number of F-16s owned by Taiwan in the US remains 10.
The four F-16s sent back to Taiwan arrived in April. They are now deployed at an air base near Hualien.
The defense source said the US provided the planes in order to comply with the terms of the original deal between the two nations, which included a commitment to provide replacement aircraft to maintain the Taiwanese air force's F-16 combat readiness ratio -- calculated by dividing the total number of F-16s by the number of planes ready for combat.
"According to the contract signed by the US and Taiwan, the US has an obligation to help Taiwan's air force maintain its combat readiness ratio of the F-16s at 70 percent or above," the source said.
"A series of F-16 crashes in 1998 and 1999 caused some problems in the maintenance of the combat readiness ratio," he said.
An officer assigned to Taiwan's air force general headquarters, who asked not to be named, said the combat readiness ratio of the F-16s is very high.
In related news, sources say the US has shipped a large quantity of F-16 parts, as well as Maverick and Sparrow missiles.
Part shortages are said to be a common problem in the air force, especially parts needed to its advanced fighter planes -- the F-16 and Mirage 2000-5 fighter planes -- in top form and ready for combat.
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