Apparently the Russians did so in a provocative manner
A Russian Su-27 Flanker fighter jet intercepted on January 25 a US Air Force RC-135U spy plane over the Black Sea, a spokeswoman for the US Department of Defense said.
Lt. Col. Michelle L. Baldanza said the US aircraft was on its regular route in the international airspace over the Black Sea when the Russian Su-27 fighter jet intercepted it in "an unsafe and unprofessional manner."
She added that all details of the incident involving the Russian and US Airforce aircraft were thoroughly studied and investigated.
US Fox News also reported about the incident over the Black Sea. FoxNews.com cited Pentagon Press Secretary Peter Cook as saying that Russian military officials discussed during a video conference with Pentagon on Thursday flight safety over Syria "to avoid accidents and unintended confrontation between coalition and Russian forces whenever the two sides operate in close proximity."
However, Baldanza said that "No specific incident or event prompted today's discussion."
The Washington Free Beacon daily reported that the Russian fighter "came within 20 feet of a US RC-135 reconnaissance aircraft over the Black Sea on Monday."
Citing US defense officials the daily reported that "the Su-27 flew alongside the RC-135, an electronic intelligence-gathering aircraft, and then performed what they said was an aggressive banking turn away from the intelligence jet."
"The thrust from the Su-27 ‘disturbed the controllability’ of the RC-135, said one official familiar with details of the incident," the daily added.
Source: TASS - Russian news agency