Turkey scrambled jets after a Russian surveillance plane flew along its Black Sea coast and a U.S. warship passed through Turkey's Bosphorus straits on its way to the Black Sea, although the U.S. military said it was a routine deployment.
The head of Russia's upper house of parliament said after meeting visiting Crimean lawmakers on Friday that Crimea had a right to self-determination, and ruled out any risk of war between "the two brotherly nations".
European Union leaders and Obama said the referendum plan was illegitimate and would violate Ukraine's constitution. Obama called German Chancellor Angela Merkel from his Florida vacation on Friday to discuss the situation in Ukraine.
"The leaders reiterated their grave concern over Russia's clear violation of international law through its military intervention in Ukraine," the White House said in a statement.
Obama ordered visa bans and asset freezes on Thursday against so far unidentified people deemed responsible for threatening Ukraine's sovereignty.
Earlier in the week, a Kremlin aide said Moscow might refuse to pay off any loans to U.S. banks, the top four of which have around $24 billion in exposure to Russia.
Japan endorsed the Western position that the actions of Russia constitute "a threat to international peace and security," after Obama spoke to Prime Minister Shinzo Abe.
(Read more: Vitali Klitschko: Putin worried over Ukraine)
China, often a Russian ally in blocking Western moves in the U.N. Security Council, was more cautious, saying economic sanctions were not the best way to solve the crisis and avoiding comment on the Crimean referendum.
The EU, Russia's biggest economic partner and energy customer, adopted a three-stage plan to try to force a negotiated solution but stopped short of immediate sanctions.
The Russian Foreign Ministry responded angrily on Friday, calling the EU decision to freeze talks on visa-free travel and on a broad new pact governing Russia-EU ties "extremely unconstructive." It pledged to retaliate.
Senior Ukrainian opposition politician Yulia Tymoshenko, freed from prison after Yanukovich's overthrow, met Merkel in Dublin and appealed for immediate EU sanctions against Russia, warning that Crimea might otherwise slide into a guerrilla war.
Brussels and Washington rushed to strengthen the new authorities in economically shattered Ukraine, announcing both political and financial assistance. The regional director of the International Monetary Fund said talks with Kiev on a loan agreement were going well and praised the new government's openness to economic reform and transparency.
The European Commission has said Ukraine could receive up to 11 billion euros ($15 billion) in the next couple of years provided it reaches agreement with the IMF, which requires painful economic reforms such as ending gas subsidies.
Promises of billions of dollars in Western aid for the Kiev government, and the perception that Russian troops are not likely to go beyond Crimea into other parts of Ukraine, have helped reverse a rout in the local hryvnia currency.
In the past two days it has traded above 9.0 to the dollar for the first time since the Crimea crisis began last week. Local dealers said emergency currency restrictions imposed last week were also supporting the hryvnia.
Russian gas monopoly Gazprom said Ukraine had not paid its $440 million gas bill for February, bringing its arrears to $1.89 billion and hinted it could turn off the taps as it did in 2009, when a halt in Russian deliveries to Ukraine reduced supplies to Europe during a cold snap.
In Moscow, a huge crowd gathered near the Kremlin at a government-sanctioned rally and concert billed as being "in support of the Crimean people." Pop stars took to the stage and demonstrators held signs with slogans such as "Crimea is Russian land" and "We believe in Putin."