Russia is unhappy the U.S. is selling antitank weapons to Kiev.
https://www.wsj.com/articles/javelin-missiles-for-ukraine-1514237429
https://newtimes.ru/articles/detail/137734
https://www.gazeta.ru/army/2017/12/26/11548646.shtml
https://www.gazeta.ru/business/2017/12/26/11542952.shtml
https://www.gazeta.ru/business/2017/12/26/11547152.shtml
http://www.rosbalt.ru/russia/2017/12/26/1671312.html
https://www.rbc.ru/newspaper/2017/12/26/5a3b920b9a79477856403329