U.S. Defense Secretary Ashton Carter refused to transfer four key technologies needed to build Korea’s KF-X fighter jets despite Korean National Defense Minister Han Min-koo’s appeals Thursday.

Han accompanied President Park Geun-hye on her first visit to the Department of Defense during a four-day Washington trip leading up to a summit with U.S. President Barack Obama.

In bilateral talks with Carter, Han asked the Pentagon chief to reconsider Washington’s rejection of an export license for key technologies from Lockheed Martin to be transferred to Korea to help develop its indigenous fighter jets over the next decade.

But the U.S. government made clear it wants to preserve its advanced F-35 stealth fighter technology, including an active electronically scanned array (AESA) multifunction radar system, for security reasons.

A Korean defense official told reporters after the talks on Thursday that “Minister Han requested a forward-looking review of the transfer of the technologies for the KF-X project, but Secretary Carter adhered to the position that it would be difficult to transfer the four technologies for the KF-X, even conditionally.”

The official added, “Secretary Carter, however, said that he will think of ways for joint cooperation in technological cooperation [for the KF-X project].”

This includes forming a new consultative body for Korea and U.S. defense technology and industry cooperation, he said.

In a statement, the Pentagon said that the two sides agreed to “establish an interagency working group to enhance cooperation on defense technology issues” without elaborating on specifics.

Carter’s refusal is the third rejection by the U.S. for the transfer of the four core technologies to Korea.

In September 2014, the Korean government signed a 7.34 trillion won ($6.41 billion) deal with U.S. defense and aerospace company Lockheed Martin to buy 40 F-35A jets and also receive technical support for Korea’s project to locally build its own next-generation fighter jet.

But it was only last month that Korea’s Defense Acquisition Program Administration finally admitted that in April, Washington had refused licenses to transfer to Korea four core technologies in F-35 stealth fighter jets that are needed to develop KF-X aircraft.

Han sent a letter to Carter in August requesting the transfer of the technologies, and there was no response until this week. The Pentagon finally sent a response via the U.S. Embassy in Korea, which delivered it to the Ministry of National Defense in Seoul Thursday morning, even though Han was already in Washington.

From the very start, Korean arms negotiators were aware that it would be difficult to get U.S. government approval for the four core technologies, although it would receive 21 of 25 technologies from Lockheed. Even so, they went through with the costly deal with Lockheed last September.

The technologies include the AESA radar system designed to detect targets faster and more precisely at longer ranges than existing radar in Korea’s fighter jets, an infrared search and track system, an electronic optics targeting pod and a radio frequency jammer.

The government has been scrambling to find alternative sources for the technologies to see through its ambitious 8.1 trillion won “Boramae Project” to develop 120 indigenous mid-level KF-X fighter jets by 2025.

The main opposition New Politics Alliance for Democracy on Friday called for a comprehensive review and parliamentary audit of the KF-X project.

“President Park accompanied by Defense Minister Han during the U.S. visit tried to find a way out for the KF-X project, but the bubble has popped on our government’s plan,” said NPAD floor spokesman Park Soo-hyun.