The United States on Thursday imposed visa bans and set the stage for wider sanctions against Russia, warning any move to split the Crimean peninsula from Ukraine would break international law.
President Barack Obama spoke to Russian President Vladimir Putin to explain the measures, which he said were in response to Russia's "violation of Ukraine's sovereignty and territorial integrity."
Obama further stiffened the US response to Russia's incursion into Ukraine as his Secretary of State John Kerry worked in Europe for a diplomatic way out of the worst East-West crisis in decades.
The visa ban targets Russians and Ukrainians blamed for threatening the sovereignty or territorial integrity of Ukraine, where pro-Moscow president Viktor Yanukovych was ousted after mass protests last month.
- Pressuring Putin -
They were also merely a first step. Obama also signed an executive order paving the way for economic sanctions against individuals or entities in Russia.
The document sets broad criteria, and could target those accused of usurping peace and stability in Ukraine or Russian officials seeking to impose control over any part of the country.
"These decisions continue our efforts to impose a cost on Russia and those responsible for the situation in Crimea," said Obama.
"And they also give us the flexibility to adjust our response going forward based on Russia’s actions."
In practice, the sanctions may offer a way to crank up pressure on Putin by restricting the assets or movement of senior figures around the Russian president.
Those stripped of their visas would be informed, US officials said, refusing to make the list public.
But State Department spokeswoman Jen Psaki suggested Putin was not on it.
"Taking a step like that against a head of state is -- would be a significant step, and it is not what we are leading with here in this process," she said.
Obama also rejected a bid by the Crimean parliament to join Russia -- a move seen as a way for Putin to cement control over the region.
"The proposed referendum on the future of Crimea would violate the Ukrainian constitution and violate international law," he said.
"In 2014, we are well beyond the days when borders can be redrawn over the heads of democratic leaders."
Pro-Moscow officials on the Crimean peninsula announced on Thursday they would hold a referendum on March 16 on whether to join the Russian Federation.
But Washington says only the people of Ukraine could make decisions about their borders.
"You can't have a situation in which the legitimate government of the country is excluded from decision-making about different parts of that country. That is clearly a violation of international law," a senior official said.
The White House said the Putin call lasted for an hour.
"President Obama indicated that there is a way to resolve the situation diplomatically, which addresses the interests of Russia, the people of Ukraine, and the international community," a White House statement said.
In Moscow, the Kremlin said Putin had told Obama that relations between the two countries "should not be sacrificed due to disagreements over individual -- albeit extremely significant -- international problems."
Obama has worked over the last week to build a united front among the United States and its allies on Ukraine, and the US sanctions coincided with new European pressure on Putin.
US officials stressed however that, despite the deployment of Russian troops into southern Ukraine's Crimea peninsula, Washington had not seen any military moves into mainland eastern Ukraine.
In a bid to defuse the tensions on Europe's doorstep, Kerry, who visited Kiev on Tuesday, held a second round of talks in two days with his Russian counterpart Sergei Lavrov.
But Lavrov told Russian news agencies that so far "we cannot report to the international community that we are in agreement."
In Washington, the House of Representatives overwhelmingly passed a bill providing loan guarantees to the interim Ukrainian government.
Lawmakers voted 385 to 23 in favor of the bill, but the Senate is not expected to take up the matter until next week.
The White House has already said it will offer $1 billion in loan guarantees and is pressing Congress to act swiftly.
Washington could also help Kiev by reducing its dependence on Russian gas supplies after Moscow threatened to hike the prices, said lawmaker Ed Royce.
- An off ramp -
Angered by the show of Russian might, Washington has already pulled out of preparatory meetings for the G8 summit in Sochi. Other steps include suspending bilateral discussions on trade and investment.
While upping consequences for Russia over its move into Ukraine, Obama also left open what US officials have been calling an off ramp for Putin.
He called for a diplomatic solution that would see Russian troops pulled back to barracks in Crimea, elections in Ukraine in May, international monitors to safeguard the rights of ethnic Russians in the region and retain Moscow's military basing rights on the peninsula.