A Russian Su-27 fighter jet came close to a US reconnaissance plane RC-135 during its operation over the Baltic Sea on Monday, Fox News reported.

US Secretary of Defense James Mattis stated last week that the chairman of the US Joint Chiefs of Staff, Gen. Joseph Dunford, was in talks with his Russian counterpart Gen. Valery Gerasimov to ensure that the two countries’ military operations in the Baltic Sea region are deconflicted.

Since June 2 there have been more than 35 interactions in the Baltic Sea region between U.S. and Russian jets and warships, but the incident Monday morning is notable because the U.S. military considered it “unsafe,” according to one official.

On June 6, the Russian Defense Ministry said it scrambled a Su-27 air superiority fighter over the Baltics to to intercept and escort a US B-52 strategic bomber that approached its borders.

On May 12, the ministry said Moscow scrambled a Su-30 fighter plane on May 9 over the Black Sea to intercept a US reconnaissance aircraft.