US Democratic Senator Chris Van Hollen stated that Turkish President Recep Tayyip Erdogan had crossed “another red line on the S-400s” by starting to test the missile systems’ radars.

“Two weeks after his WH visit, Erdogan is thumbing his nose at [US President Donald] Trump, the U.S. + NATO”, Van Hollen tweeted on Monday, urging Trump to slap sanctions on Ankara.
The statement comes after Erdogan stated earlier in the day that efforts to resolve the S-400 row with the US may continue until April.
“There is a process that is ongoing until April. Our defence and foreign ministers will carry out these efforts. We need to see where we get with these efforts,” Erdogan said.
His remarks were preceded by the Milliyet daily citing an unnamed source as saying that the Turkish military will fly its F-16 fighter jets near the capital Ankara to test the S-400’s radar systems on Monday and Tuesday.
The issue of the S-400 was on the table during the talks between Erdogan and Trump in Washington last week, when the US President admitted that Turkey’s purchase of the Russian-made missile systems “creates some very serious challenges” for the US.
He added that the two leaders had tasked their national security advisers with “immediately” getting to work on resolving the issue.
He spoke after a senior US State Department official Turkish authorities last week to stop using the S-400s, claiming that “there is room for Turkey to come back to the table” to get Ankara back into the F-35 stealth fighter programme.
“They know that to make this work they need to either destroy or return or somehow get rid of the S-400,” the official added.
Despite Washington’s move to suspend Ankara’s participation in the F-35 program over its purchase of the S-400s, Turkish Defence Minister Hulusi Akar recently stated that his country is going to activate and make use of the S-400 air defences.
“The S-400 will be functioning. At the moment, Turkish military personnel are undergoing training on the use of the S-400s. After this is concluded, the planned [use of the S-400s] will start. Do not doubt it,” he added.
The US has repeatedly objected to Ankara’s purchase of the S-400s, saying that the missile systems are incompatible with NATO security standards and may compromise the operations of its fifth-generation F-35 fighter jets.
Ankara rejects the allegations, insisting that the S-400s were not related to the security of NATO, the US, or the F-35 in any way.