Two Russian strategic bombers conducted practice cruise missile attacks on the United States during a training mission last week that defense officials say appeared timed to the NATO summit in Wales.

The Russian Tu-95 Bear bombers were tracked flying a route across the northern Atlantic near Iceland, Greenland, and Canada’s northeast.

Analysis of the flight indicated the aircraft were conducting practice runs to a pre-determined “launch box”—an optimum point for firing nuclear-armed cruise missiles at U.S. targets, said defense officials familiar with intelligence reports.

Disclosure of the nuclear bombing practice comes as a Russian general last week called for Moscow to change its doctrine to include preemptive nuclear strikes on the United States and NATO.

Gen. Yuri Yakubov, a senior Defense Ministry official, was quoted by the state-run Interfax news agency as saying that Russia’s 2010 military doctrine should be revised to identify the United States and the NATO alliance as enemies, and clearly outline the conditions for a preemptive nuclear strike against them.

Yakubov said among other needed doctrinal changes, “it is necessary to hash out the conditions under which Russia could carry out a preemptive strike with the Russian Strategic Rocket Forces”—Moscow’s nuclear forces.

The practice bombing runs are the latest in a series of incidents involving threatening Russian bomber flights near the United States. Analysts say the bomber flights are nuclear saber-rattling by Moscow as a result of heightened tensions over the crisis in Ukraine.

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