Turkey has begun building a platform site for a Russian S-400 missile defense system, despite pressure from Washington to abandon plans to purchase the military equipment, CNBC reported on Thursday, citing a source with firsthand knowledge of an unpublished intelligence assessment.

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.

S 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.

In December 2017, Russia and Turkey signed a loan agreement to supply S-400s to Ankara. According to a statement by the Turkish defense industry secretariat, two S-400 batteries will be operated and serviced by the Turkish military.

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.