The White House on Sunday warned Russia that it will come under increased international pressure if it presses ahead with a referendum to annex Crimea.
Deputy National Security Advisor Tony Blinken said on CNN that pressure on Russia will only go up if the referendum goes forward on March 16.
"First, if there is an annexation of Crimea, a referendum that moves Crimea from Ukraine to Russia, we won't recognize it, nor will most of the world," Blinken said.
"Second, the pressure that we've already exerted in coordination with our partners and allies will go up. The president made it very clear in announcing our sanctions, as did the Europeans the other day, that this is the first step and we've put in place a very flexible and very tough mechanism to increase the pressure, to increase the sanctions," he said.
Crimean offcials say they will go ahead with a vote next Sunday to join the Russian Federation, but international diplomatic efforts are intensifying against the move.
On Sunday German Chancellor Angela Merkel spoke with Turkish Prime Minister Tayyip Erdogan. Both leaders agreed that Ukraine's territorial integrity needed to be protected at all costs. A statement from the German government said a referendum planned for March 16 on Crimea joining Russia was "extremely dubious" and "illegal."
The Reuters news agency reported that Merkel and Erdogan said efforts to form an "international contact group" and a committee to investigate violent incidents of recent weeks were important.
According to Reuters, Erdogan said Turkey was prepared to help the international contact group given his country's close relationships with Ukraine and Russia, as well as its special relationship and contact with the Crimean Tatars.
Also on Sunday Russian President Vladimr Putin spoke by telephone with German Chancellor Angela Merkel and British Prime Minister David Cameron and said the steps taken by authorities in Ukraine's Crimea region were in accordance with international law.
President Obama will meet with Ukrainian Interim Prime Minister Arseny Yatsenyuk at the White House on Wednesday to discuss efforts to resolve the crisis.
Obama said last week that any referendum on Crimea would violate international law and the Ukrainian constitution. He also announced sanctions including travel bans and the freezing of assets of individuals responsible for Russia's military intervention in Crimea. Putin was not among the individuals.
Ukraine's acting defense minister says Kyiv has no plans to send armed forces to Crimea. The Interfax news agency quoted acting Defense Minister Ihor Tenyukh as saying on Sunday that Ukrainian troops are performing training exercises, but they are strictly limited, involving only troop movements from one base to another.
"No movements, no departures for Crimea by the armed forces are foreseen. They are doing their routine work which the armed have always had," he said. Tenyukh was responding to media reports about Ukrainian troop movements after Russian forces took control of Crimea.
Ukraine's leaders vowed Sunday not to give up "a single centimeter" of territory to Russia as thousands rallied at rival pro- and anti-Moscow demonstrations, and tensions remained high over the deepening crisis in Crimea.
Ukraine's acting head of government Yatsenyuk led commemorations in the capital, Kyiv, for the 200th anniversary of the birth of Ukraine's most revered poet and national hero, Taras Shevchenko.
He told a crowd that the country's "fathers and grandfathers have spilled their blood for this land. And we won't budge a single centimeter."
Meanwhile, rallies honoring Shevchenko were held in Crimea's key cities of Simferopol and Sevastopol, the eastern city of Donetsk and other areas. The rallies were against the breakup of Ukraine. Pro-Russia protesters held counter-demonstrations in cities throughout Crimea and eastern Ukraine. Scattered clashes were reported between the two sides.
One of the speakers at a rally in Kyiv, until recently imprisoned Russian tycoon and Putin critic Mikhail Khodorkovsky, held back tears as he implored the crowd to believe that not all Russians support their country's recent actions in Ukraine.
Russian forces tightened their grip on Crimea on Sunday. In the latest armed action, Russians took over a Ukrainian border post on the western edge of Crimea, trapping about 30 personnel inside.
A Ukrainian military spokesman, Oleh Slobodyan, said Russian forces now control 11 border guard posts across Crimea.
Russia denies it has troops on the peninsula beyond those regularly stationed with its Sevastopol-based Black Sea fleet. Ukraine's much smaller navy is also based in the Crimean port city.
Witnesses say although the soldiers have no insignia identifying them, they are clearly Russian.
Foreign observers have failed to get into Crimea to get a first-hand look at the situation and were forced to turn back Saturday after pro-Kremlin gunmen fired warning shots.
Unease in Crimea continues after Ukraine's acting president, Oleksandr Turchynov, signed a decree Friday canceling a March 16 referendum on Crimea joining Russia. Local authorities in Crimea say the ballot will go forward.
Russian lawmakers have vowed to support Crimea's decision if the Ukrainian region decides to join Russia. On Sunday, a Russian lawmaker said the Kremlin had set aside $1.1 billion to rebuild Crimea's industrial infrastructure if the disputed region votes to join Russia.