The U.S. government has refused to allow Korea to export $400 million worth of indigenous supersonic training jets to Uzbekistan, sources said Sunday.

The denial of permission is another blow to Korea’s program to develop its own military aircraft and comes at a sensitive time when the U.S. government’s refusal to hand over key avionics technologies regarding F-35 fighters to Korea has become a political issue between the two nations.

Korea Aerospace Industries’ (KAI) T-50 Golden Eagle was co-developed in 2006 with Lockheed Martin, using the U.S. firm’s core technologies, including the avionics system and engine.

Because of this, Korea needs to get approval from the U.S. government to export the aircraft in accordance with the U.S. Arms Export Control Act.

“KAI has been in negotiations with the Uzbek government to export the supersonic trainers, but the U.S. government is opposing the deal, citing possible technology leakage and diplomatic policy,” a source said.

The U.S. refusal comes at a sensitive time when the Korea’s weapons procurement agency has drawn fire for failing to receive key avionics technologies from its purchase of 40 F-35 stealth jets as an offset package.

KAI seeks to sell 12 T-50s worth $400 million (454 billion won).

In addition, the U.S. also expressed concerns that Uzbekistan’s procurement of the T-50s may ratchet up tensions with neighboring countries, the source said.

The military believes that the U.S. opposition is due to Uzbekistan’s membership of the Shanghai Cooperation Organization (SCO) that some say has emerged as an anti-U.S. bulwark in Central Asia. The SCO’s other members are China, Kazakhstan, Kyrgyzstan, Russia and Tajikistan.

“As Uzbekistan has close ties with Russia, the U.S. is worried that an export of T-50s to Uzbekistan may lead to its technologies being transferred to Russia,” said a military official.

U.S.-Russia ties are at their lowest ebb in decades since Moscow’s annexation of the Crimean Peninsula from Ukraine in March 2014.

The U.S. opposition is a major blow to Korea, which has made all-out efforts to strike the contract with the Central Asian country.

President Park Geun-hye and Uzbek President Islam Karimov reportedly discussed the issue during their summit at Cheong Wa Dae in May.

In April, Defense Minister Han Min-koo and his Uzbek counterpart Kabul Berdiev signed the memorandum of understanding on promoting bilateral partnerships and Berdiev piloted the FA-50 simulator. The FA-50 is a light attack variant of the T-50 that has been exported to Iraq, the Philippines and Thailand. KAI also exported 16 TA-50s, another T-50 variant, to Indonesia.

However, the Defense Acquisition Program Administration (DAPA), the nation’s arms procurement agency, is still seeking to keep the talks with Uzbekistan alive.

“Despite the U.S. opposition, DAPA will step up efforts to persuade Washington to approve the deal,” the military official said.

KAI, based in Sacheon, South Gyeongsang Province, is also promoting the supersonic trainer for the U.S. Air Force’s trainer program, codenamed “T-X,” aimed at replacing its fleet of T-38s, in 2017.

The U.S. plans to purchase 350 new aircraft worth billions of dollars.