The U.S. Defense Department has delivered a report to the U.S. Congress detailing the implications of Turkey receiving 100 F-35 fighter jets, five people familiar with the report said.

Relations between Ankara and Washington have been going through a turbulent period over Turkey's purchase of Russian S-400 defense missile systems, a decision that raised concerns among some NATO member countries.

In early August, the administration of U.S. President Donald Trump approved a defense budget law delaying delivery of F-35 Joint Strike Fighter jets to Turkey, citing Turkey's purchase from the Russians.

An amendment to the National Defense Authorization Act (NDAA), quickly approved by Congress, prohibited F-35 sales to Turkey until the Pentagon issued a report on Turkish-American relations in 90 days.

On the other hand, officials from Ankara have repeatedly said that the purchase of the S-400 systems was prompted to fulfill the country's security needs.

Turkey's planned purchase of the Russian-made S-400 missile defense system has raised concerns in the West, since it could be used to give Moscow deep insight into the vulnerabilities of the most advanced U.S. warplane at a time of tension between the two powers, Reuters quoted experts as saying.

Ellen Lord, the Pentagon's chief arms purchasing official, told Reuters in an interview that Turkey's plans to buy the S-400 system were "extremely problematical" and numerous U.S. officials had discussed the issue with Ankara, but there were no signs that Turkey had changed its mind about buying the Russian system.

Turkey last month said it was moving ahead with the S-400 procurement and will start installing anti-aircraft missile systems in October 2019.

Lord said the report to Congress "just lays out the facts of where we are," rather than offering firm recommendations, but she declined to provide details.

"We need to work with Congress to decide where we go on that. There will be a strong partnership with Congress, and until we've discussed the issue with them," Lord told Reuters in an interview on the sidelines of a NATO industry conference in Berlin earlier this week.

Still, the F-35 Joint Program Office announced on Aug. 16 that it would continue to execute current program plans with all partners, including Turkey, and to abide by any future policy guidelines.

Turkey is an important partner in the international F-35 program and is the only source in the world for some of the warplane's parts. Moreover, according to recent reports, halting the delivery of new generation F-35 fighter jets to Turkey could cause serious problems for its maker, Lockheed Martin.

Ten companies from Turkey, one of the global leaders in aerospace manufacturing, will make about $12 billion worth of parts for the F-35, including key components such as the center fuselage and some landing gear.

Previous reports also indicated that if the U.S. Congress blocks the sale of F-35s to Turkey, Ankara could stop supplying crucial parts for the jet.

The country plans to purchase 100 F-35 fighter jets in the coming years. Out of 100 aircraft, 30 have been approved. The country took delivery of its first F-35 fighter jet at a ceremony in Fort Worth, Texas, on June 21. The first F-35 aircraft delivered is also the first fifth-generation fighter jet to enter the Turkish Air Force Command inventory.

Turkey is due to receive its third and fourth jets in March next year. Its pilots are receiving training on the first two aircraft at Luke Air Force Base in Arizona. The earliest the first aircraft could leave the United States is expected to be next summer.