Russia’s decision to send long-range strategic bombers on regular patrol missions across the Gulf of Mexico is unprecedented, a senior U.S. military official said Wednesday, claiming that the country has never done so before – even during the Cold War.

The official, who spoke to The Associated Press on condition of anonymity because he wasn't authorized to discuss the flights publicly, also said that the pace of Russian flights around North America, including the Arctic, have largely remained steady, with about five incidents per year.

Long-range bombers have been in the area before, but only to participate in various visits to the region when the aircraft stopped over night at locations in South or Central America. During the Cold War, other types of Russian aircraft flew patrols there, including surveillance flights and anti-submarine aircraft.

Col. Steve Warren, a Pentagon spokesman, declined to call this a Russian provocation.

“The Russians have patrolled in the Gulf [of Mexico] in the past and we’ve seen the Russian Navy operate in the Gulf of Mexico,” he said Wednesday. “These are international waters. It’s important that the Russians conduct their operations safely and in accordance with international standards.”

The announcement by Russian Defense Minister Sergei Shoigu – who also stated that the patrols would occur elsewhere around the globe -- came as NATO's chief accused Russia of sending fresh troops and tanks into eastern Ukraine.

"Over the last few days, we have seen multiple reports of large convoys moving into Eastern Ukraine," said NATO Secretary-General Jens Stoltenberg. "We assess that this significant military buildup includes Russian artillery, tanks, air defense systems and troops. His statement called the situation a "severe threat to the cease-fire."

Moscow denied the allegation as unfounded, but Shoigu also said the dispute with the West over Ukraine would require Russia to beef up its forces in the Crimea, the Black Sea Peninsula that Russia annexed in March.

Shoigu said Russian long-range bombers will conduct flights along Russian borders and over the Arctic Ocean. He said, "In the current situation we have to maintain military presence in the western Atlantic and eastern Pacific, as well as the Caribbean and the Gulf of Mexico."

Shoigu would not say how frequent the patrol missions would be or offer any other specifics, but he noted that the increasing pace and duration of flights would require stronger maintenance efforts and that relevant directives have been issued to industries.

He said the Russian air force's long-range planes also will conduct "reconnaissance missions to monitor foreign powers' military activities and maritime communications."

Russian nuclear-capable strategic bombers were making regular patrols across the Atlantic and the Pacific Oceans during Cold War times, reaching areas from which nuclear-tipped cruise missiles could be launched at the United States. But that stopped in the post-Soviet economic meltdown.

The bomber patrol flights have resumed under President Vladimir Putin's tenure, and they have become even more frequent in recent weeks, with NATO reporting a spike in Russian military flights over the Black, Baltic and North seas as well as the Atlantic Ocean.

Earlier this year, Shoigu said that Russia plans to expand its worldwide military presence by seeking permission for navy ships to use ports in Latin America, Asia and elsewhere for replenishing supplies and doing maintenance. He said the military was conducting talks with Algeria, Cyprus, Nicaragua, Venezuela, Cuba, Seychelles, Vietnam and Singapore.

Shoigu said Russia also is talking to some of those countries about allowing long-range bombers to use their air bases for refueling.

Ian Kearns, director of the European Leadership Network, a London-based think tank, said the bomber patrols are part of Kremlin's efforts to make the Russian military "more visible and more assertive in its actions."

The new bomber flights "aren't necessarily presaging a threat," Kearns said. "They are just part of a general ramping-up of activities."

But, he added, "The more instances you have of NATO and Russian forces coming close together, the more chance there is of having something bad happening, even if it's not intentional."

On Monday, the European Leadership Network issued a report that found a sharp rise in Russian-NATO military encounters since the Kremlin's annexation of Crimea, including violations of national airspace, narrowly avoided midair collisions, close encounters at sea, harassment of reconnaissance planes, close overflights over warships, and Russian mock bombing raid missions.

Three of the nearly 40 incidents, the think tank said, carried a "high probability" of causing casualties or triggering a direct military confrontation: a narrowly avoided collision between a civilian airliner and a Russian surveillance plane, the abduction of an Estonian intelligence officer, and a large-scale Swedish hunt for a suspected Russian submarine that yielded no result.

In September, the report said, Russian strategic bombers in the Labrador Sea off Canada practiced cruise missile strikes on the U.S. Earlier this year, in May, the report said, Russian military aircraft approached within 50 miles of the California coast, the closest such Russian military flight reported since the end of the Cold War.

Russia-West ties have dipped to their lowest point since Cold War times over the Kremlin's annexation of Crimea and support for pro-Russia insurgents in Ukraine. The West and Ukraine have continuously accused Moscow of fueling the rebellion in eastern Ukraine with troops and weapons — claims Russia has rejected.

Fighting has continued in the east, despite a cease-fire agreement signed between Ukraine and the rebels signed in Minsk, Belarus, in September.

Stoltenberg, the NATO chief, urged Russia to "pull back its forces and equipment from Ukraine, and to fully respect the Minsk agreements. "

U.S. Air Force Gen. Philip Breedlove, the Supreme Allied Commander in Europe, said Wednesday that in the last two days "we have seen columns of Russian equipment, primarily Russian tanks, Russian artillery, Russian air defense systems and Russian combat troops entering into Ukraine."

Breedlove, who spoke in Sofia, Bulgaria, wouldn't say how many new troops and weapons have moved into Ukraine or specify how the alliance obtained the information.

The Russian Defense Ministry quickly rejected Breedlove's statement as groundless.

Breedlove said the Russia-Ukraine border is "completely wide open," and "forces, money, support, supplies, weapons are flowing back and forth."

The Associated Press contributed to this report.