Russian developed surface-to-air missile system.
1Turkey has threatened to retaliate if Washington imposes sanctions over the S-400.
2The deliberation follows signs that Turkey is moving ahead with the S-400 purchase.
3A senior U.S. official said the purchase of the S-400 risked triggering sanctions.
4The bill says Trump should implement sanctions on Turkey over the S-400 purchase.
5Russian S-400 anti-aircraft technology is considered some of the most sophisticated.
6The Pentagon, separately, said that the S-400 should not be activated.
7Some of its hardware, such as its S-400 air defense systems, is world-class.
8He reiterated Turkey 's desire to buy U.S. Patriot defenses in addition to the S-400s.
9Both Saudi Arabia and Qatar have reportedly been in talks to buy the S-400.
10According to Ankara, the S-400s do not pose a direct threat to the U.S. military.
11Turkey seems to be moving ahead with the S-400 purchase, regardless of the U.S. warnings.
12After Ankara bought the S-400s, Washington suspended its involvement in the program and threatened sanctions.
13But sanctions over S-400s, now back on the radar, are making some optimists think twice.
14An apparent S-400s test last week prompted a furious response from the State Department and Pentagon.
15Turkey shrugged off threats of U.S. sanctions and began receiving its first S-400 deliveries in July.
16Still, Turkey seems to be moving ahead with the S-400 purchase, regardless of the U.S. warnings.