Member of the US Senate Democrat Chris van Hollen said that Turkey crossed the red line by starting testing the radars of the S-400 air defense systems acquired from Russia, he wrote about this on his Twitter account, RIA Novosti reports.

“Two weeks after his WH visit, Erdogan is thumbing his nose at Trump, the U.S. + NATO, and crossing another red line on S-400s.

Existing law requires Trump to impose sanctions. Pompeo must also confront Turkey about its latest ‘safe zone’ violations and attacks against the Kurds,” he tweeted.

As a result of talks in Washington with Erdogan, Trump said that the parties instructed their officials to work to solve the existing problems around the S-400 systems.

Earlier, the Milliyet newspaper reported, citing a military source, that Turkey is beginning to test the operation of S-400 radars, for this purpose F-16 fighters and other aircraft will be lifted into the air.

Deliveries of the latest Russian S-400 air defense systems, which caused a crisis in relations between Turkey and the US, began in mid-July. According to Turkish President Tayyip Erdogan, the S-400 will be fully operational in April 2020. Washington demanded a refusal of the deal and in exchange to buy the American Patriot systems, threatening to delay or even cancel the sale of the latest F-35 fighters to Turkey, as well as impose sanctions in accordance with CAATSA (Countering America's Adversaries Through Sanctions Act). Ankara refused to make concessions.