President Barack Obama on Friday warned against Russian intervention in Ukraine, as Russian troops landed in the Crimean capital.
(For the latest developments, click here for more coverage from NBC News).
Obama said any Russian military intervention in Ukraine "would represent a profound interference in matters thatare the business of (the) Ukranian people."
The president made his statement shortly following reports of the Russian military troop movements in Crimea. The Ukrainian government was reported to have characterized the deployment as an invasion.
US Ambassador to the United Nations Samantha Power separately said the United States was "gravely disturbed" by the reports.
Rumors about possible Russian action in the Ukraine were cited as a potential factor in stocks pulling back late in the session Friday.
(Read more: Russia likely to get what Russia wants in showdown)
"That is something we're watching very carefully; it appears that Russia will take an active role in the Ukraine," said Jim Russell, senior equity strategist at US Bank Wealth Management, who cited the potential for political disruptions to the natural gas and "to some degree global oil output."
US officials confirmed to NBC that uniformed Russian forces, apparently a hybrid of military and paramilitary, were flying into the Crimean capital Simferopol.
While not in position to confirm the numbers of troops, the US officials told NBC they have no reason to doubt the basic information the troops are Russians arriving on Russian aircraft.
The troops may be moving beyond the airport.
—Additional reporting and writing by CNBC.com staff and by Reuters.