Putin has defied calls from the West to pull back his troops, and Moscow has insisted that it has a right to protect its interests and ensure the welfare of the many ethnic Russians in Crimea and elsewhere in Ukraine amid the instability following the recent protests and change of power.
However, there has been no sign of ethnic Russians facing attacks in Crimea, where they make up about 60 percent of the population, or elsewhere in Ukraine.
Russian border guard services said about 675,000 Ukrainians had left for Russia in the months of January and February, and said it was a sign of "humanitarian catastrophe" according to Russia's TASS news agency.
On the road from Sevastopol — the Crimean port where Russia has its key Black Sea naval base under an agreement with Ukraine — to Simferopol, journalists with The Associated Press on Sunday saw 12 military trucks carrying troops and an armored vehicle equipped with a machine gun.
U.S. President Barack Obama spoke with Putin by telephone for 90 minutes on Saturday and expressed his "deep concern" about "Russia's clear violation of Ukrainian sovereignty and territorial integrity," the White House said. Obama warned that Russia's "continued violation of international law will lead to greater political and economic isolation."
The State Department announced that Secretary of State John Kerry will travel to Kiev Tuesday to meet with the leaders of the new government and members of the Rada, Ukraine's parliament. In announcing the trip, State Department spokeswoman Jen Psaki said Kerry would reaffirm U.S. support for Ukrainian "sovereignty, independence, and territorial integrity."
Earlier, Kerry warned Russia of "very serious repercussions" from the West for what he called an "incredible act of aggression." Possible steps including sanctions that would "isolate Russia economically" were being considered, Kerry said, along with possible visa bans.
Senior U.S. officials have reportedly been discussing a diplomatic "offramp" with Russia, allowing international observers, perhaps from the U.N. or the Organization for Security and Co-operation in Europe, to deploy to Crimea to allay Moscow's stated concerns over the safety of pro-Russia residents. Officials stressed the the U.S. is examining diplomatic and economic options for the region; not military intervention.
On Monday, British Foreign Secretary William Hague called the Russian intervention in Ukraine the biggest crisis in Europe so far this century.
Hague said that Russia now had operational control of Ukraine's Crimea region and that while Russia had the legal right to base troops in the region, the Kremlin should order them to return to their barracks.
"Clearly we are very concerned about any possibly of a further move by Russia in other parts of Ukraine but that does not mean the position in the Crimea is stable," Hague told the BBC in an interview.
"This is a very tense situation and dangerous situation that Russia's intervention has now produced."
Secretary Kerry, in an interview Sunday with Bob Schieffer of CBS's Face the Nation, also cautioned that Russia may be risking it's G-8 membership.
"If Russia wants to be a G-8 country, it needs to behave like a G-8 country," Kerry said.
The other seven countries in the G-8 partnership issued a statement on Sunday saying Russia was violating Ukraine's sovereignty. They said the countries were suspending activities leading up to the G-8 summit in Sochi for the time being.
The United States said Saturday that it will suspend participation in preparatory meetings for the Group of Eight economic summit planned for June at the Russian Black Sea resort of Sochi, site of the just-concluded Winter Olympics. The United Kingdom also said it will not take part in the Sochi summit.
President Obama on Sunday spoke with British Prime Minister David Cameron, German Chancellor Angela Merkel and Polish President President Bronislaw Komorowski in separate phone calls. All three leaders expressed "grave concern over Russia’s clear violation of Ukrainian sovereignty," and pledged financial assistance to the country, according to a White House statement.
The U.N. said Deputy Secretary-General Jan Eliasson was on her way to Ukraine.
French Foreign Minister Laurent Fabius said on French radio Europe that planning for the summit should be put on hold. France "condemns the Russian military escalation" in Ukraine, and Moscow must "realize that decisions have costs," he said.
But the U.S. and other Western governments have few options to counter Russia's military moves.
NATO's North Atlantic Council, the alliance's political decision-making body, and the NATO-Ukraine Commission were set to meet on Sunday. NATO Secretary-General Anders Fogh Rasmussen said the allies will "coordinate closely" on the situation in Ukraine, which he termed "grave."
NATO Secretary General Anders Fogh Rasmussen called on Russia to withdraw its military forces back to their bases and refrain from interfering elsewhere in Ukraine.
Ukraine is not a NATO member, meaning the U.S. and Europe are not obligated to come to its defense. But Ukraine has taken part in some alliance military exercises and contributed troops to its response force.
Ukraine's acting president, Oleksandr Turchynov, announced late Saturday that he had ordered his armed forces to be at full readiness because of the threat of "potential aggression." He also said he had ordered stepped-up security at nuclear power plants, airports and other strategic infrastructure.
In Crimea, however, Ukrainian troops have offered scant resistance to the Russian incursion.
The new government in Ukraine came to power last week following months of pro-democracy protests against the now-fugitive Yanukovich, and his decision to move Ukraine politically and economically closer to Russia instead of the European Union.
Ukraine's population of 46 million is divided in loyalties between Russia and Europe, with much of western Ukraine advocating closer ties with the EU, while eastern and southern regions look to Russia for support. Crimea, a semi-autonomous region that the then-Soviet Union granted Ukraine in the 1950s, is mainly Russian-speaking.
Al Jazeera and wire services