More On Ukraine

The president threatened the action in response to Russia's seizure of three Ukrainian ships and the sailors aboard.

The Su-27UB Flanker-C experienced "an emergency situation that brought it to a catastrophe."

Lt. Col. Seth "Jethro" Nehring and Ukrainian pilot Col. Ivan Petrenko were killed in the crash.

The airman was a member of the California Air National Guard's 144th Fighter Wing, U.S. Air Forces Europe-Africa said.

The pilot was reportedly aboard the Ukrainian fighter when it crashed.

Russia has amassed as many as 40,000 troops and dozens of fighter jets, helicopters, tanks and armored vehicles near Ukraine.

Mattis on Friday met with Ukraine's Minister of Defence Stepan Poltorak at the Pentagon.

Mattis said that the U.S. decision to supply "defensive" weapons to Ukraine should be no cause of alarm for the Russians.

The DoD said any decisions on the sale of anti-tank and anti-aircraft weapons would come from the State Department.