Russian transport planes and helicopters flew into Crimea on Friday, and a warship blocked Ukrainian coast guard vessels, raising new concerns about military intervention by Moscow.
Armored personnel carriers were seen on the roads and troops secured the main military airport, where more Russians were flying into the autonomous region in southern Ukraine.
Ukraine's U.N. ambassador denounced the border crossing as illegal but said the country, just days after a power shift that ousted Russian-friendly president Viktor Yanukovych, could handle the display of military might.
"We are strong enough to defend ourselves,'' Ambassador Yuriy Sergeyev told reporters after briefing the U.N. Security Council in a closed-door session.
Earlier, President Barack Obama warned that “there will be costs to any military intervention in Ukraine,”
At the U.N., Russian Ambassador Vitaly Churkin insisted the movements were "within the framework" of Moscow's pre-existing agreement with Ukraine on military presence in the former Soviet republic.
House Intelligence Committee Chairman Mike Rogers said it appeared to him that the Russian military now controls the Crimean peninsula.
"This aggression is a threat not only to Ukraine, but to regional peace and stability," Rogers said. "Russia's latest action is yet another indicator that Vladimir Putin's hegemonic ambitions threaten U.S. interests and allies around the world."
Reuters contributed to this report.