Republican James Lankford and Democrat Chris Van Hollen wrote to Secretary of State Mike Pompeo asking about the report and saying that Washington's failure to act more decisively about the S-400 purchase had "emboldened" Turkey's government.
Turkey bought a batch of the missile systems from Russia last year, leading to its suspension by Washington from the US F-35 stealth fighter jet program. The United States has said Turkey risks US sanctions if it deploys the Russian-made S-400s, but has not yet imposed them.
The State Department did not immediately respond to a request for comment on the letter.
Bloomberg reported on Tuesday that Turkey was planning to conduct a comprehensive test of the S-400 missile-defense system, citing people familiar with the matter.