S-400

The president previously described a construction project in Istanbul as "a little conflict of interest" in his dealings with Turkey.

Turkey began radar testing of its Russian-made S-400 anti-aircraft system on Monday, the purchase of which has created friction with Washington.

Relations between NATO allies the U.S. and Turkey have been strained by Ankara's decision to purchase the Russian-made S-400 anti-aircraft system.

Turkey's purchase of the Russian anti-aircraft system has opened up a rift with the U.S., which fears the deal could compromise American military secrets.

Washington's demand that Turkey kill a deal for Russia's S-400 anti-aircraft system was "unacceptable," a Kremlin spokesman said.

The S-400 system is considered the most advanced in the world.

"Any effort by the Government of the Republic of Turkey to further enhance their relationship with Russia will degrade the general security of the NATO alliance."

The Russian S-400 system is effective against stealth aircraft, ballistic missiles and cruise missiles.

Moscow is studying the remains of Tomahawk rockets to exploit their weaknesses.

Air Force Secretary Heather Wilson said U.S. JSTARS command and control planes would not survive initial contact with Russian and Chinese defenses.

The Israeli prime minister is in Moscow to discuss ties with Russia and celebrate the defeat of Nazi Germany.

Moscow has repeatedly hinted it could send some of its most advanced air defense systems to Syria.

The world's top two military powers may be on a collision course in Syria.

Russia has bolstered its air defenses in Syria, but shooting down a U.S. cruise missile is a sophisticated task that will put Putin's military to the test.

Defense experts have warned that the U.S. must upgrade its artillery forces.

The U.S. has threatened to impose sanctions on Turkey in a bid to stop it buying the S-400 from Russia.

China, Egypt, India, Iraq, Saudi Arabia, Sudan, Syria and Turkey were among those looking to get their hands on the S-400 missile defense system.

As Russia consolidates its position in Syria and the Middle East as a whole, it's reaching out to rivals and selling them a coveted weapon, the S-400.

Turkey boasts the second largest army in NATO but its government is growing ever closer to Russia.

Poland has long called for U.S. backing in deterring Moscow, and Russian missiles are set to arrive near its border.

Moscow had its S-400 missile system in range of the strike.

Rostec says it logged thousands of suspicious cyber-incidents last year.