ISTANBUL, (Reuters) -President Tayyip Erdan said Sunday that the United States offered to sell F-16 fighter planes to Turkey as a return on its investment in F-35. This was after Ankara had purchased missile defense systems from Russia.

Reuters earlier reported that Turkey requested the purchase of 40 Lockheed Martin F-16 fighter planes as well as nearly 80 modernization kit for its warplanes.

Erdogan stated to reporters that Turkey is seeking a return on its investment in F-35, and that negotiations are ongoing.

Erdogan stated that $1.4 billion was paid for F-35s. The U.S. offered such a payment in exchange for those payments.

“And concerning this, we said lets take whatever steps necessary to meet our defense needs,” he stated, noting that F-16 aircraft would aid in developing the nation’s fleet.

Ankara has ordered over 100 F-35 aircraft, which were made by Lockheed Martin Corp (NYSE :), however, after Russia acquired the S-400 missile defense system systems from Moscow in 2019, the U.S. retracted Turkey’s participation.

In the last five years, the decades-old alliance between NATO allies was rocked by disagreements over Syria policy, Ankara’s close ties to Moscow, NATO’s naval ambitions at the eastern Mediterranean, U.S. accusations against an Turkish state-owned bank, and the erosion of rights in Turkey.

U.S. sanctions have also been triggered by Ankara’s purchase S-400s. Washington sanctioned Turkey’s Defence Industry Directorate in December 2020. The chief of the department, Ismail Demir was blacklisted along with three other employees.

Since that time, the U.S. have repeatedly warned Turkey to avoid buying more Russian weaponry. Erdogan however indicated Ankara plans to continue buying S-400s from Russia. This move could damage Washington’s relationship.

American Congress will not approve of the request to fly the planes. The Congress has shown a deep dislike for Turkey in recent years.

Bipartisan support exists in the U.S. Congress for pushing Ankara’s Biden government to increase pressure, principally over Russia’s purchase of weapons and its record on human rights.

Ankara stated that it hopes for stronger ties under the U.S. President Joe Biden.