WASHINGTON (AP) -- The State Department is warning U.S. citizens to put off all nonessential travel to Ukraine, and particularly Crimea.
The department cited "the potential for instability" following the ouster of Viktor Yanukovych as president and the establishment of a new government.
Groups have staged protests, set up roadblocks and occupied government buildings in several cities throughout Ukraine since November 2013. The State Department warned all U.S. citizens in Ukraine, and those considering travel there to be aware of the possibility of violence.
President Barack Obama spoke of "reports" of military movements inside Ukraine, and officials said the U.S does believe that Russia is intervening.
Ukrainian officials say armed men described as Russian troops took control of key airports in Crimea on Friday and Russian transport planes flew into the strategic region.
Obama could pull Russia trip amid Ukraine tumult
WASHINGTON (AP) -- The Obama administration is evaluating whether President Barack Obama will go forward with plans to attend an international summit in Russia this summer amid reports of Russian intervention in Ukraine.
A senior administration official says it's hard to see how Obama and European leaders would attend the G-8 summit in Sochi, which is scheduled for June. The official says the administration is also evaluating whether to proceed with discussions with Russia on deepening trade and commerce ties.
The official outlined the potential consequences of Russian intervention shortly after Obama declared there would be "costs" to such activity. Obama did not outline those costs during his Friday afternoon statement at the White House.
The official was not authorized to discuss the situation by name and spoke only on condition of anonymity.
Ukraine official: 8 Russian cargo planes in Crimea
KIEV, Ukraine (AP) -- A spokesman for the Ukrainian border service says eight Russian transport planes have landed in Crimea with unknown cargo.
Serhiy Astakhov tells The Associated Press that the Il-76 planes arrived unexpectedly Friday and were given permission to land, one after the other, at Gvardeiskoye air base, north of the regional capital, Simferopol.
Astakhov says the people in the planes refused to identify themselves and waved off customs officials, saying they didn't require their services.
Earlier Friday, Ukraine's U.N. ambassador said he had told the U.N. Security Council that Russian military helicopters and transport planes are entering his country and that Russian armed forces seized Crimea's main airport.
Russia's Interfax agency cited Serhyi Kunitsyn, a Ukraine presidential envoy to Crimea, telling ATR television that 13 Russian planes carrying 150 Russian troops each landed at Gvardeiskoye air base. That report could not be confirmed.
Ukraine's acting president has urged Russian President Vladimir Putin to stop "provocations" in Crimea and pull back military forces from the peninsula.
Oleksandr Turchynov, who stepped in as president after Viktor Yanukovych fled Kiev last weekend, said Friday that the Ukrainian military will fulfill its duty but will not be drawn into provocations.