WASHINGTON, November 26. /TASS/. Turkey has crossed ‘another red line’ by starting tests of the radar detection system it purchased from Russia as part of the S-400 missile defense system, US Senator for Maryland Christopher Van Hollen said.

"Two weeks after his WH [White House] visit, [Turkish President Recep Tayyip] Erdogan is thumbing his nose at Trump, the U.S. + NATO, and crossing another red line on S-400s," Van Hollen said in a Twitter post, attaching an article on the issue.

"Existing law requires Trump to impose sanctions. Pompeo must also confront Turkey about its latest ‘safe zone’ violations and attacks against the Kurds," the US senator added.

The Pentagon refused to comment on whether the US personnel was taking part in Turkey’s trials of S-400 systems, which it had earlier began with the use of US-made F-16 fighter jets.

Turkish President Recep Tayyip Erdogan said on November 19 he told US President Donald Trump that Ankara would not give up the Russian-made S-400 systems.

Russia announced in September 2017 that it had signed a $2.5 billion deal with Turkey on the delivery of S-400 anti-aircraft missile systems to Ankara. Under the contract, Ankara will get a regiment set of S-400 air defense missile systems (two battalions). The deal also envisages partial transfer of production technology to the Turkish side.

Turkey is the first NATO member state to purchase such air-defense missile systems from Russia. The deliveries of S-400 air defense systems to Turkey began on July 12, 2019.

The United States and NATO attempted to prevent Turkey from purchasing Russia's S-400 missile systems. Washington warned on many occasions that it may impose sanctions on Turkey, if Ankara presses ahead with the S-400 deal.