The source said the assessment was conducted about a month ago and includes satellite imagery of a concrete launch facility and other constructions, including bunkers, the US media reported.
The source also indicated that the work at the site "fits the pattern" for Russia's S-400 system, the report said.
The report gave no indication about where the site is located in Turkey.
READ MORE: No Decision on Sanctions Waiver for India for Purchase of Russian S-400 — Pompeo
US officials have warned that if Ankara goes ahead with plans to purchase the Russian S-400 anti-missile systems, the United States could withhold the transfer of F-35 jets. The US Congress also recently passed legislation that would block the transfer of F-35s if Turkey buys the Russian S-400s.
Last week, Turkish Prime Minister Recep Tayyip Erdogan confirmed that Turkey expects to receive the Russian S-400 missile defense systems in the near future.