The incident happened right after Russia said it would treat all US jets in certain areas over Syria as targets.
A Russian Su-27 fighter jet intercepted a US RC-135 reconnaissance plane in a "provocative" and "unsafe" manner on Monday over the Baltic Sea, two US officials told Fox News.
The Russian jet came within 5 feet of the US aircraft, Fox News said.
The US plane was flying in international airspace when it was intercepted by the Russian jet, which was armed, an Air Force spokesperson told Business Insider.
The spokesperson could not determine exactly where or at what time the incident happened, but United Press International said it occurred about 25 miles from the Russian city of Kaliningrad.
"Due to the high rate of closure speed and poor control of the aircraft, the interaction was deemed unsafe," the spokesperson said.
"Russia is within its rights to operate in international airspace, but there are international standards to ensure safety and prevent incidents," the spokesperson said.
The incident happened not long after Russia said it would track US and US-led coalition that flew over its combat-mission zones in Syria "as air targets" after a US jet shot down a pro-Syrian regime fighter jet on Sunday.
The incident also happened after the 45th annual BALTOPS military exercise, in which the US and 13 other western countries participated, over the Baltic Sea from June 1-16.
The exercise involved "4,000 shipboard personnel, 50 ships and submarines and more than 50 aircraft, [and] is designed to enhance flexibility and interoperability, to strengthen combined response capabilities, as well as demonstrate resolve among Allied and Partner Nations’ forces to ensure stability in, and if necessary defend, the Baltic Sea region," the Navy said in a statement.