NATO headquarters spokesman Lt. Col. Jay Janzen said one aircraft based in England would observe Russian air and sea movements from Polish air space, while the other based in Germany would fly over Romania. Both Poland and Romania are NATO members and border Ukraine, and Romania's Black Sea coast is only about 220 kilometers (140 miles) from the Crimean peninsula.
Janzen said the planes - both Boeing E-3 Sentry aircraft that sport a rotating radar dome above the fuselage - would be able to monitor military movements covering an area of 300,000 square kilometers (115,000 square miles) and willl not leave NATO air space.
"Regardless, we can observe, we can look, a very long way," he said.
The Sentry is also known as AWACS, short for "airborne warning and control system," and is the main battlefield command and surveillance aircraft for NATO air forces.
The 28-nation NATO alliance decided Monday to use AWACS to monitor Russia's military buildup, and the first Sentry sortie over Romania happened Tuesday, Janzen said.
The U.S. Air Force already has deployed extra combat fighters to NATO bases in Eastern Europe, including six F-15s last week in Lithuania and a dozen F-16s this week in Poland.
Meanwhile, a U.S. Navy destroyer joined Bulgarian and Romanian naval forces in the Black Sea for exercises a few hundred miles off the Crimean peninsula.
The drills on Wednesday include the guided-missile destroyer USS Truxton, the Bulgarian naval frigate Drazki and three Romanian vessels.
Bulgaria's Defense Ministry said in a statement that the drills were planned in 2013 and were in no way related to the recent events in the Ukraine.
Grieshaber reported from Berlin.