Between March 2015 and November 2018, the Pentagon spent about $300 million to fly aerial refueling missions in support of Saudi Arabia, the United Arab Emirates and others engaged in the nearly eight-year conflict.
Putin's declaration paves the way for Russia to openly send troops and weapons to the long-running conflict pitting Ukrainian forces against Moscow-backed rebels.
The United Arab Emirates is the first to use the Terminal High Altitude Area Defense System in an attack by Houthi militants. It successfully took out a mid-range ballistic missile aimed at a civilian site in the country.