Turkey's defense minister on Wednesday objected to a letter sent by the U.S., saying it "is not in line with the spirit of alliance".

Speaking to reporters in Gebele, Azerbaijan, Hulusi Akar said: "We have noticed [...] that the manner is not in line with the spirit of alliance. We are accordingly preparing a response."

U.S. Acting Defense Secretary Patrick Shanahan sent a letter to Akar last week saying Washington will cut short a training program on F-35 fighter jets for Turkish pilots over "safety concerns".

"We've suspended some of the activities in terms of training. We haven't suspended any of the maintenance activity," the letter said.

Akar reiterated that Turkey has always remained committed to its responsibility toward NATO and the U.S.

"To date, we have fulfilled all of our responsibilities without any delay or interruption toward the U.S. and NATO," he said.

Akar also said that he will speak to Shanahan over the phone on Thursday and meet at the NATO Defense Ministers Meeting in Brussels by the end of June.

Tensions between the U.S. and Turkey have lingered on over the purchase of Russian S-400 defense systems by Ankara.

Turkey decided in 2017 to purchase the S-400 system following protracted efforts to purchase air defense systems from the U.S. with no success.

Washington maintains the move will jeopardize Turkey's role in the U.S. F-35 fighter jet program and could trigger congressional sanctions.