An armed Russian fighter jet flew within 5ft (1.5m) of a US plane in a "provocative" manoeuvre over the Baltic Sea, it has been reported.
The Sukhoi Su-27 jet approached an American RC-135 reconnaissance plane "rapidly" and flew "erratically", according to officials quoted by Fox News.
CBS said the fighter approached the wingtip of the larger aircraft before passing underneath it and reappearing by the opposite wing.
The US considers the incident, which took place some 25 miles north-west of Kaliningrad, an unsafe intercept.
Officials told Fox the Russian jet was armed with air-to-air missiles.
It was reportedly unclear how close the RC-135 was to Kaliningrad, a Russian enclave sandwiched between Poland and Lithuania, both Nato members.
The Russian defence ministry said that a US reconnaissance RC-135 plane had swerved dangerously in the proximity of a Russian Su-27 fighter jet over the Baltic Sea, RIA news agency reported.
It said the Su-27 had also intercepted another RC-135 plane in the area.
Russia, a close ally of the Assad regime, said its surface-to-air missile systems would track "all kinds of airborne vehicles, including aircraft and UAVs of the international coalition detected to the west of the Euphrates River" as targets.
The US said the Syrian plane, an Su-22, had targeted an American-allied, Kurdish-led force that is battling the IS extremists in their de facto capital, Raqqa.
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