ISTANBUL — Defying strenuous American objections and the threat of sanctions, Turkey began receiving the first shipment of a sophisticated Russian surface-to-air missile system on Friday, a step certain to test the country’s uneasy place in the NATO alliance.

The system, called the S-400, includes advanced radar to detect aircraft and other targets, and the United States has been unyielding in its opposition to Turkey’s acquisition of the equipment, which is deeply troubling to Washington on several levels.

It puts Russian technology inside the territory of a key NATO ally — one from which strikes into Syria have been staged. The Russian engineers who will be required to set up the system, American officials fear, will have an opportunity to learn much about the American-made fighter jets that are also part of Turkey’s arsenal.

That is one reason the Trump administration has already moved to block the delivery of the F-35 stealth fighter jet, one of the United States’ most advanced aircraft, to Turkey, and has suspended the training of its pilots, who were learning how to fly it. (Whether NATO, in turn, might glean some Russian secrets from Turkey’s acquisition of the S-400 is unclear.)