US President Barack Obama has warned Russia that the West will be forced to apply a cost to Moscow if it fails to change course in its dispute with Ukraine.
Mr Obama held face-to-face talks with new Ukrainian Prime Minister Arseny Yatsenyuk in the Oval Office today.
With Mr Yatsenyuk seated at his side, he said: "We will stand with Ukraine."
Mr Yatsenyuk said Ukraine stands ready for talks on the crisis, and vowed to "never surrender" to Russia.
The European Union has agreed a framework for its first sanctions on Russia since the Cold War.
It is a stronger response to the Ukraine crisis than many had expected and a mark of solidarity with the United States in the effort to make Moscow pay for seizing Crimea.
The EU sanctions, outlined in a document seen by Reuters, would slap travel bans and asset freezes on an as-yet-undecided list of people and firms accused by Brussels of violating the territorial integrity of Ukraine.
German Chancellor Angela Merkel said the measures would be imposed on Monday unless diplomatic progress was made.
Russian troops have seized control of the Black Sea peninsula.
Separatists have taken over the provincial government and are preparing for a referendum on Sunday to make the region part of Russia, which the West calls illegal.
The measures outlined by the EU are similar to steps already announced by the US.
However, the EU measures would have far greater impact because Europe buys most of Russia's oil and gas exports, while the US is only a minor trade partner.
The EU's €335bn of trade with Russia in 2012 was worth around ten times that of the US.
The travel bans and asset freezes could cut members of Russia's elite off from the European cities that provide their second homes and the European banks that hold their cash.
The fast pace of Russian moves to annex Crimea appears to have galvanised the leaders of a 28-member bloc whose consensus rules often slow down its decisions.
Ms Merkel herself had initially expressed reservations about sanctions but has been frustrated by Moscow's refusal to form a "contact group" to seek a diplomatic solution over Crimea.
"Almost a week ago, we said that if that wasn't successful within a few days, we'd have to consider a second stage of sanctions," Ms Merkel said.
"Six days have gone by since then and we have to recognise, even though we will continue our efforts to form a contact group, that we haven't made any progress."
Referendum preparations in Crimea
In Crimea, the regional government is led by a Russian separatist businessman whose party received just 4% of the vote in the last provincial election in 2010, but who took power on 27 February after gunmen seized the assembly building.
Two days later, Russian President Vladimir Putin announced that Moscow had the right to invade Ukraine to protect Russian citizens.
Preparations for Sunday's referendum are in full swing.
Crimea has a narrow ethnic Russian majority, and many in the province of 2m people clearly favour rule from Moscow.
Opinion has been whipped up by state-run media that broadcast exaggerated reports of a threat from "fascist thugs" in Kiev.
But a substantial, if quieter, part of the population still favours being part of Ukraine.
They include many ethnic Russians as well as Ukrainians and members of the peninsula's indigenous Tatar community, who were brutally repressed under Soviet rule.
The referendum seems to leave no such choice: voters will have to pick between joining Russia or adopting an earlier constitution that described Crimea as sovereign.
The regional assembly says that if Crimea becomes sovereign, it will sever ties with Ukraine and join Russia anyway.
There is little doubt that the separatist authorities will get the pro-Russian result they seek.
Many opponents, including Tatar leaders, plan to boycott.
There will be no Western observers. Election officials have openly said they proudly support union with Russia.
Journalists seeking accreditation for the vote are required to promise not to report "negative news".
In a statement, the leaders of the G7 - the US, Britain, France, Germany, Italy, Japan and Canada - called on Russia to stop the referendum from taking place.
"In addition to its impact on the unity, sovereignty and territorial integrity of Ukraine, the annexation of Crimea could have grave implications for the legal order that protects the unity and sovereignty of all states," they said.
"Should the Russian Federation take such a step, we will take further action, individually and collectively."
While tightening his grip on Crimea, Mr Putin seems to have rowed back from his 1 March threat to invade other parts of eastern and southern Ukraine, where most of the population, though ethnically Ukrainian, speak Russian as a first language.
The authorities in Kiev announced the formation of a new national guard today.