Two Russian strategic bombers circled the U.S. island of Guam last week in what U.S. defense officials say is the latest in a series of nuclear provocations by Moscow.

The bombers were identified by air defenses as Tu-95 Bear H nuclear-capable aircraft that circumnavigated the strategic U.S. military outpost on Friday—amid heightened tensions with Moscow regarding a new buildup of Russian forces in and along the border of eastern Ukraine.

"U.S. Pacific Command can confirm that two aircraft circumnavigated Guam on November 13th," said spokesman Maj. Christian Devine. "The aircraft were flying safely in international airspace and in accordance with international norms; as such, the decision was made to not intercept them."

Friday’s flights were the second time in the past two years that Russia conducted unusual long-range bomber missions around the island. Two Tu-95s circled the island on Feb. 12, 2013 and were intercepted by F-15 jets.

The latest bomber flights around the island, located 4,000 miles west of Hawaii, appeared timed to coincide with the G-20 summit in Brisbane, Australia and come amid new tensions with Russia over Ukraine.

The flights took place a day before President Obama arrived in Brisbane for the summit where he met with Russian President Vladimir Putin.

The president told reporters Sunday he had a "businesslike and blunt" exchange with Putin on Ukraine, and urged the Russian leader to "resolve the issue." Russia’s military forcibly annexed Ukraine’s Crimea and is threatening military action in support of pro-Moscow rebels in eastern Ukraine.

The flights also came after Moscow announced plans to conduct long-range strategic bomber flights over the Gulf of Mexico and the Caribbean. Russian Defense Minister Sergey Shoigu said Nov. 12 that "we have to maintain [Russia's] military presence in the western Atlantic and eastern Pacific, as well as the Caribbean and the Gulf of Mexico," adding that dispatches of bombers would be "part of the drills."

Shoigu said the flights are "connected to the situation in Ukraine, with the emerging anti-Russia inclinations on the part of NATO and the increasing foreign military presence in the immediate vicinity of our borders."

Guam is the U.S. military’s most important strategic hub in the Asia Pacific and the key base for the Pentagon’s new "Air Sea Battle" concept that calls for closer coordination of Air Force and Navy forces to counter China.

The Air Force regularly deploys B-52 and B-2 bombers to Guam and the long-range Global Hawk drone also operates from the island.

The War is Boring blog reported Nov. 9 that the new RQ-170 Sentinel spy drone was photographed at the island’s Andersen Air Force Base.

The Navy has deployed three attack submarines to Guam and plans to add a fourth as part of the so-called Asia pivot.

The 2013 Russian bomber incursion around Guam came shortly before President Obama’s state of the union message.

Russia in recent years has stepped up aerial incursions into U.S. air defense zones in both the western and eastern coasts. One bomber incursion near Canada’s east coast was assessed by U.S. intelligence agencies as a practice for a nuclear cruise missile strike on the United States.

The closest encounter came in June when two bombers came within 50 miles of California in what military officials said was the nearest Russian strategic aircraft had flown since the Cold War.

"The nuclear bomber provocations have been going on since 2007 but clearly have reached an unprecedented level in 2014," said Mark Schneider, a former Pentagon official involved in strategic nuclear policy.

"We had better start taking these activities and Russian nuclear strategy seriously and take actions that enhance our nuclear deterrent, or we risk a serious miscalculation on Putin’s part that could result in a major war," said Schneider, now with the National Institute for Public Policy.

Propeller-driven Bear H bombers are relatively old aircraft but are equipped with Kh-55SM cruise missiles. The missile can be armed with nuclear or conventional warheads and have a range of up to 1,800 miles.

Putin was given a harsh welcome at the Brisbane summit, which ended Sunday. Canadian Prime Minister Stephen Harper told Putin at the meeting, "I guess I’ll shake your hand, but I have only one thing to say to you: You need to get out of Ukraine."

Putin left the summit early but denied the departure was related to foreign leaders’ opposition to Russian intervention in Ukraine.

During brief encounters at an earlier summit in Beijing and the one in Australia, Obama told Putin that Russia should abide by the September peace agreement reached in Belarus on Ukraine and voiced concerns about a Russian military buildup in eastern Ukraine, White House Press Secretary Josh Earnest told reporters Nov.13.

On Sunday in Australia, Obama accused the Russians of violating the Belarus accord by supplying arms to rebels in eastern Ukraine.

The Bear bomber flights followed earlier aerial provocations over Europe in late October, prompting NATO interceptor jets to shadow Russian bombers as they flew along Europe’s northern coast as far south as Portugal.

The Russian aircraft were supposed to fly through the Mediterranean where they would have been refueled by aerial tankers that had been deployed to Egypt’s Cairo International Airport. But the longer flights were called off, U.S. and NATO officials said.

Adm. John Kirby, the Pentagon’s press spokesman, voiced concerns about the Russian flights over Europe. "We certainly don’t see these increased flights and activity as helpful to the security situation in Europe," Kirby told reporters.

"Clearly they pose a potential risk of escalation."

Russia under Putin, a former KGB political police and intelligence service official, has increased its nuclear saber rattling in recent months. In August, the Russian leader warned the West not to "mess" with a nuclear-armed Russia.

Russia’s Pravda, the Russian Federation Communist Party newspaper, reported Nov. 11 that Moscow is preparing a "nuclear surprise" for NATO: A vast numerical superiority over NATO in numbers of tactical nuclear weapons.

"Here is another surprise, the report said. "As for tactical nuclear weapons, the superiority of modern-day Russia over NATO is even stronger."

The report said NATO has deployed 260 tactical nuclear arms in Europe, including 200 U.S. bombs at six airbases in Germany, Italy and Belgium.

"Russia, according to conservative estimates, has 5,000 pieces of different classes of [theater nuclear weapons]—from Iskander warheads to torpedo, aerial and artillery warheads," the report said.

UPDATED 8:45 a.m.: This story was updated to include official comment from the U.S. Pacific Command (PACOM), and corrected to reflect the PACOM spokesman's assertion that only two Russian bombers circumnavigated the island, and not four, as was initially reported.