Russia's increasing military activities around the world have unsettled top US military officials, who say they are reshaping their budget plans to better address what they now consider to be the most pressing threat to US security.

"Russia is the No 1 threat to the United States. We have a number of threats that we're dealing with, but Russia could be, because of the nuclear aspect, an existential threat to the United States," Air Force Secretary Deborah James told Reuters in an interview at the annual Reagan National Defence Forum.

James, Chief of Naval Operations Admiral John Richardson and Pentagon chief arms buyer Frank Kendall, all voiced growing concern about Russia's increasingly aggressive behaviour in interviews late on Saturday.

Their comments come as the Pentagon finalises a classified security assessment for President-elect Donald Trump, who has promised to both pump up US defence spending and build closer ties to Russian President Vladimir Putin.

European diplomats fear Moscow could use the time before Trump's inauguration to launch more offensives in Ukraine and Syria, betting that President Barack Obama will be loathe to respond forcefully so soon before he hands over power on Jan 20.

Kendall said US policy had been centred on threats in the Asia-Pacific region and Middle East, but was now focused more on Russia. "Their behaviour has caused us ... to rethink the balance of capabilities that we're going to need," he said.

None of the officials gave details about how the concerns would affect the fiscal 2018 budget request, but defence officials have pointed to the need to focus on areas such as cyber security, space, nuclear capabilities and missile defence, where Russia has developed new capabilities in recent years.

Pentagon officials have nearly completed work on a fiscal 2018 budget request, but it is likely to be reworked substantially once Trump takes office. Officials expect that budget to be submitted in April at the earliest, and possibly later. Typically, budgets are submitted in early February.

Marine Corps General Joseph Dunford, chairman of the Joint Chiefs, told the conference that Russia's goal was to counter Nato, undermine its credibility and limit the ability of the US military to project power around the world.

"They are operating with a frequency and in places that we haven't seen for decades," he said, adding that the buildup should be viewed in the context of its actions in Ukraine, Crimea and Syria, where they have already stepped up air attacks on eastern Aleppo.

Richardson said the Navy was seeing increased Russian naval activities around the globe, including its unprecedented deployment of a carrier strike group to the Mediterranean, the firing of missiles from ships in the Caspian Sea, increased submarine activities in the north Atlantic, and a growing naval presence in the Pacific.

He said there were continuing incidents involving Russian aircraft buzzing US vessels, with some coming as close as 30 feet, and other cases where ships were behaving "erratically."

"It's all for public consumption," Richardson said, noting that Russian ships often filmed such encounters and edited them to make it appear as if US ships were at fault.

Russia and the United States have an agreement to limit and discuss incidents at sea, but the accord appeared to be having little impact on curbing such incidents, he said.

Dialogue between US and Russian navy officers has ceased since Russia's annexation of the Crimea region of Ukraine in 2014, in contrast to the days of the Cold War, when US and Russian officials were in more regular contact, he said.

"More communication with Russia would be a valuable thing," Richardson said, noting that he had regular contact with his counterpart in China, but not with those in Russia or Iran.

James echoed his concerns, citing what she called "very worrying" incidents of "very dangerous airmanship" and cyber attacks by Russian hackers on US institutions.

Richardson also said he was concerned about a report by Norwegian Defence Minister Ine Eriksen Soereide that Russia now had the ability to cut off resupply routes to Europe through its activities in the Arctic region.

Army Secretary Eric Fannning told a panel at the conference that Russia was clearly acting "in a destabilising way," and said the United States was learning from how the Russian military was behaving in Ukraine.