A Russian jet flew within 5ft (1.5m) of a US reconnaissance plane's wing tip over the Baltic Sea on Monday, US officials say.
The encounter was deemed "unsafe" due to the Russian pilot's "high rate of closure speed and poor control of the aircraft", officials said.
But Russia disputes the American account, saying the US plane made a "provocative" move towards their jet.
On Monday, Russia warned that US jets flying in Syria would be targeted.
The announcement came in response to the US downing of a Syrian jet after it targeted American-allied rebels.
On Tuesday, the US military shot down an armed Iranian-made drone in Syria, adding to tensions between the Washington and Moscow, which is allied with the Syrian regime.
- Syria conflict: US jet 'downs Iranian-made drone'
The intercept on Monday occurred 25 miles (40 km) from the Russian enclave of Kaliningrad, over international waters.
Pentagon spokesman Captain Jeff Davis told reporters: "We were flying in international airspace and did nothing to provoke this behaviour."Image copyright Getty Images Image caption A Russian SU-27 fighter jet
The US RC-135 reconnaissance aircraft had been flying with its transponder on, making it visible to other aircraft, he added.
But Russia's defence ministry said it was the US plane that behaved dangerously after it was already intercepted and was being escorted by a Russian Su-27 fighter jet.
"During the escorting, the RC-135 crew made an attempt of closing with the Russian fighter, performing a provocative turnaround toward the Su-27," the TASS news agency quoted the Russian ministry as saying.
About 10 minutes later, another US surveillance plane arrived, and it too was intercepted by a SU-27 jet, the ministry said.
Earlier this month, the US Air Force deployed 800 troops and long range B-52 bombers to the United Kingdom to take place in joint Nato exercises.
There have been more than 30 interactions between US and Russian planes and ships in the Baltic Sea since the beginning of June, US officials tell CNN.
The majority have been deemed safe, US officials say.