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(CNN) —  

The US Air Force has pulled 26 Turkish military personnel from its F-35 fighter jet training program due to “safety” concerns stemming from the ongoing dispute over Turkey’s push to buy both American stealth aircraft and a Russian missile defense system, according to a defense official directly familiar with the matter.

Specifically, Air Force officials were concerned the pending decision to end Turkey’s involvement in the F-35 program this summer if it moves forward with a plan to purchase the Russian S-400 air defense system could impact the Turkish pilots’ focus, the official said.

Brigadier Gen. Todd Canterbury, the wing commander at Luke Air Force Base, made the decision on his own Friday despite there being no evidence the Turks pose a threat to the aircraft or US personnel or would be unable to concentrate on flying.

The safety concern is being explained as the same procedure used to temporarily ground US military pilots if they are facing a “significant life event,” such as a divorce or death in the family, that would be a distraction during flight operations, the official told CNN.

Lt. Col. Mike Andrews, Department of Defense spokesman, confirmed the pilots are no longer training.

“The department is aware that the Turkish pilots at Luke AFB are not flying. Without a change in Turkish policy, we will continue to work closely with our Turkish ally on winding down their participation in the F-35 program,” he said.

The Pentagon announced Friday that Turkey has until July 31 to reach a mutual agreement with the US over the purchase of the Russian S-400 missile defense, otherwise the US will block Turkey from purchasing the F-35 fighter jet and cease the training of its pilots.

In a letter to Turkish Defense Minister Hulusi Akar, acting Secretary of Defense Patrick Shanahan wrote, “Turkey’s procurement of the S-400 will hinder your nation’s ability to enhance or maintain cooperation with the United States and within NATO.”

Shanahan warned that the S-400 purchase will “lead to Turkish strategic and economic over-dependence on Russia.”

But while Shanahan suggested training would not be suspended for several weeks while the two governments try to find a resolution, Canterbury’s decision will expedite that process for 26 members of the Turkish military, includes six pilots and 20 aviation maintenance personnel, who had their access to the flight line at Luke AFB revoked.

They are being allowed to stay at the base and have full access to other areas the official said. In addition, classroom training for other Turkish personnel continues at Eglin AFB.

The defense official told CNN the decision was completely based on safety concerns and was not intended to contradict Shanahan’s statement.

US officials have long warned Turkey that it would not be allowed to acquire the F-35 stealth jet if it goes ahead with the missile system purchase.

The US believes that the Russian-system is incompatible with the F-35 jet, saying that Moscow could use it to gather intelligence on the aircraft. But Russia has consistently maintained that it intends to move forward with its deal to sell the S-400 to Turkey despite protests from the US.

Shanahan said Tuesday that he plans to address the issue during a phone call with his Turkish counterpart Wednesday.

“We have suspended some of the activities in terms of training. We haven’t suspended any of the maintenance activity. But I’m hoping tomorrow for he and I to exchange views and get an update on what kind of progress we’ve made,” he said.

Kremlin aide Yury Ushakov said Tuesday an agreement to supply Turkey with the S-400 air-defense system would proceed despite US objections, Russian state news agency RIA-Novosti reported.

“The agreements reached and agreements between Russia and Turkey in this context are being fulfilled on time,” Ushakov told reporters when asked about the possibility of supplying the S-400 to Turkey in July, RIA-Novosti reported.

Delivery was to proceed “as planned,” Ushakov said.