Intercepts between US and Russian aircraft are common, and are generally not as dangerously close.
An armed Russian jet came within five feet of a US reconnaissance plane over the Baltic Sea, CBS News national security correspondent David Martin reports.
"Due to the high rate of closure speed and poor control of the aircraft during the intercept, this interaction was determined to be unsafe", Alonso said.
Drone was perceived as threat to US troops. The RC-135 reconnaissance plane compares in size to a commercial aircraft and is used to collect all electronic transmissions from a particular area.
The incident comes amid rising tensions between the US and Russian Federation after the USA downed a Syrian jet on Sunday.
Russian Federation said on Monday it would treat USA -led coalition aircraft flying west of the River Euphrates in Syria as potential targets and track them with missile systems and military aircraft. Washington and Moscow are backing different sides in Syria's civil war, which has dragged on for more than six years.
It is the second downing of a pro-regime drone in the area and comes amid heightened tensions in the region after the US downing of a Syrian fighter jet on Sunday.
That took place near the Syrian city of Taqba, which has been the focus of fighting as US -sponsored ground forces advance on the ISIS stronghold of Raqqa.
It said that the RC-135 crew carried out a provocative turn toward Russia's Su-27, adding that the Russian pilot reacted to the maneuver and kept escorting the reconnaissance plane until it changed the direction of flight away from the Russian border.
And U.S. and North Atlantic Treaty Organisation aircraft have repeatedly had close encounters with Russian aircraft - some alarmingly close - over the last two years.