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.

A spokesman for the U.S. Northern Command and North American Aerospace Defense Command declined to comment on the bomber flights in the North Atlantic.

No U.S. or Canadian fighter jets were scrambled to intercept the Bear-H bombers since the aircraft stayed outside the North American Air Defense Identification Zone.

Additional details of the incident that took place over the Labrador Sea, the stretch of the Atlantic between Greenland and Canada’s Labrador Peninsula, could not be learned.

However, officials said it took place during the NATO summit in Wales that was held Thursday and Friday.

The summit statement criticized "Russia’s aggressive actions against Ukraine [which] have fundamentally challenged our vision of a Europe whole, free, and at peace."

In response to Russia’s actions, the alliance agreed to create a new Very High Readiness Joint Task Force in Eastern Europe that can deploy military forces in days.

"If required, they will also facilitate reinforcement of allies located at NATO’s periphery for deterrence and collective defense," the NATO statement said.

U.S. Army troops will lead an international military exercise inside western Ukraine later this month. The exercises, known as "Rapid Trident 2014," will begin Sept. 15 and include troops from several NATO and NATO-partner states, including Ukraine, Azerbaijan, Bulgaria, Canada, Georgia, Germany, Britain, Latvia, Lithuania, Moldova, Norway, Poland, Romania, Spain, and the United States.

Russian nuclear forces will conduct a large-scale exercise in mid-September, state news agencies reported.

The Tu-95 is a nuclear-capable bomber that is outfitted with six AS-15 nuclear-armed cruise missiles. The missiles have a range of over 1,800 miles.

Google Earth analysis reveals that a Tu-95 launch box located in the Labrador Sea and firing AS-15 missiles would be in range of Ottawa, New York, Washington, and Chicago, and could reach as far south as the Norfolk Naval base.

However, air-launched cruise missiles fired from that location and outside the air defense identification zone would be unable to reach Kings Bay, Georgia—the homeport for U.S. ballistic missile submarines and a key strategic nuclear target.

Mark Schneider, a former Pentagon strategic policymaker and currently senior analyst at the National Institute for Public Policy, said Russian leaders frequently issue public nuclear threats because they regard their nuclear arsenal as the main element of their great power status.

"Putin began what he called bomber ‘combat patrols' in 2007 and they continue," Schneider said. "They are designed to intimidate as well as practice nuclear bomber attacks."

Schneider said that since the Ukraine crisis triggered by Moscow's military annexation of Crimea, "there have been substantial numbers of all types of standard Russian nuclear threats."

He said the threats have included nuclear exercises, bomber flights, and public statements, including Putin's suggestion that NATO ‘not mess with us' because Moscow remains a nuclear power.

Northern Command has confirmed that Russian strategic bomber flights increased sharply over the past six months.

Last month, at least 16 bomber incursions by the Russians took place within the northwestern U.S. and Canadian air defense zones over a period 10 days. It was the largest number of incursions since the end of the Cold War. U.S. fighter jets intercepted the Russian aircraft and followed them until they exited the defense zone.

In June, Russian bombers flew over the arctic prompting intercepts by Canadian fighters on two occasions. The Canadian government called the stepped up bomber flights a "strategic message" from Moscow amid heightened tensions.

And on June 20, the Russian Defense Ministry announced the test launch of six AS-15 missiles from a Bear bomber during military exercises.

That same month, on June 9, two Russian Bear bombers flew within 50 miles of the California coast in the closest strategic bomber flights near a U.S. coast since the Cold War with the Soviet Union.

Admiral Cecil Haney, commander of the U.S. Strategic Command, which is in charge of nuclear forces, said last month that he is concerned both by large-scale Russian nuclear exercises and by increased bomber flights near the United States.

"Clearly, we at the U.S. Strategic Command do monitor the strategic environment," Haney said noting large-scale nuclear exercises during the Ukraine crisis.

"Any nation state has the right to train," he added. "It’s just interesting how that information [on nuclear forces exercises] is readily available on YouTube. Clearly, the actions associated with Ukraine are problematic."

On long-range strategic aircraft flights, Haney said: "I will say that the business of them coming close to the United States of America, we take very seriously."