"Russia is the No. 1 threat to the United States," Air Force Secretary Deborah James told Reuters in an interview at the annual Reagan National Defense Forum, according to Reuters.
"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," she said.
James, Chief of Naval Operations Admiral John Richardson and Pentagon chief arms buyer Frank Kendall, all voiced growing concern about Russia's increasingly aggressive behavior in interviews late on Saturday.
Their comments come as the Pentagon finalizes a classified security assessment for President-elect Donald Trump, who has promised to both pump up U.S. defense 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 loathe to response forcefully so soon before he hands off power on Jan. 20.
Read alsoUkraine should watch out for Russian retaliation over missile drills - analystMarine 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 U.S. 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.
Dialogue between U.S. 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 U.S. and Russian officials were in more regular contact, he said.
Army Secretary Eric Fannning told a panel at the conference that Russia was clearly acting "in a destabilizing way," and said the United States was learning from how the Russian military was behaving in Ukraine.
