FILE PHOTO: A Lockheed Martin F-35 aircraft is seen at the ILA Air Show in Berlin, Germany, April 25, 2018. REUTERS/Axel Schmidt/File Photo

BERLIN/WASHINGTON (Reuters) - The U.S. Defense Department has delivered a report to Congress detailing implications of Turkey receiving 100 F-35 fighter jets, five people familiar with the report said, removing a key hurdle to concluding the deal.

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, experts have said.

Ellen Lord, the Pentagon’s chief arms buyer, 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.

The United States has for years offered Turkey an alternative missile defense system - the Patriot missile defense system built by Raytheon Co and operated by other NATO allies. However, a sale has proven elusive amid cost and technology transfer issues.

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.

Turkey last month said it was moving ahead with the controversial S-400 procurement and expected to begin installing the surface-to-air missile systems in October 2019.

The United States has repeatedly warned Turkey that going through with the purchase of S-400s could result in Washington imposing sanctions and halting other weapons deals, such as the F-35, but Ankara has pressed on with the Russian transaction.

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 next summer, although it may take longer than that.