Russia has increased tensions with the US with a show of military force.
Russian military aircraft have buzzed a US Navy destroyer in the Black Sea in incidents described as "unsafe and unprofessional" by US officials.
Russian Su-24 attack jets flew at low altitude over the USS Porter on Saturday (NZ time). Another incident involved a IL-38 jet.
"USS Porter queried all aircraft and received no response," US European Command spokesperson Captain Danny Hernandez said.
"Such incidents are concerning because they can result in accident or miscalculation."
Another US official said a Su-24 flew within 180 metres of the Porter at an altitude of 90 metres.
However the Russian Defense Ministry has denied the events occurred.
"There were no incidents of any kind on Feb. 10, related to flights by Russian military jets in the Black Sea near the US Navy destroyer Porter," a spokesman for the Russian Defense Ministry, Major-General Igor Konashenkov, is reported as saying.
US defence officials also confirm a Russian spy ship has been spotted off the US East Coast near Washington DC on Tuesday (NZ time).