Russia has launched new military exercises near its border with Ukraine, showing no sign of backing down on plans to annex Crimea despite a stronger than expected drive for sanctions from the EU and United States.
In an unusually robust and emotional speech, German Chancellor Angela Merkel warned of "catastrophe" unless Russia changes course.
Ms Merkel removed any suspicion that she might try to avoid a confrontation with Russian President Vladimir Putin,
"We would not only see it, also as neighbours of Russia, as a threat. And it would not only change the European Union's relationship with Russia," she told parliament.
"No, this would also cause massive damage to Russia, economically and politically."
US Secretary of State John Kerry said serious steps would be imposed on Monday by the US and Europe if a referendum on Crimea joining Russia takes place on Sunday as planned.
Ms Merkel, a fluent Russian speaker who grew up in Communist East Germany, has emerged in recent days as a leading figure in threatening tough measures against Moscow.
Her foreign minister, Frank-Walter Steinmeier, said European states would draw up a list over the weekend of Russians who will face visa restrictions and asset freezes.
Mr Putin declared Russia's right to invade its neighbour on 1 March, as Russian troops were already seizing control of Crimea, a Black Sea peninsula with a narrow ethnic Russian majority and a Russian naval base.
Events have moved rapidly, perhaps signalling an effort by Moscow to turn the annexation into a fait accompli before the West could co-ordinate a response.
In the Ukrainian city of Donetsk, a man was killed and more than a dozen people were in hospital after pro-Russian and pro-European demonstrators clashed.
The violence was the worst since last month's overthrow of Moscow-backed president Viktor Yanukovych.
But in an apparently conciliatory move, Russia backed deployment of an OSCE monitoring mission in Ukraine, including Crimea, the Swiss chairman of the European rights watchdog said.
The leader of pro-Moscow separatist politicians, who took power in Crimea after armed men seized the regional parliament on 27 February, predicted a strong vote in favour of union with Russia in Sunday's referendum.
#Crimea referendum official says polling stations will be open from 8am-8pm on Sunday, there will be exit polls but no indication yet when— Tony Connelly (@tconnellyRTE) March 13, 2014
"We have a survey by renowned Ukrainian and Crimean polling experts showing clearly and plainly that more than 80% of people in Crimea are ready to join the Russian Federation," Crimean prime minister Sergei Aksyonov told Reuters.
Mr Aksyonov, whose election in a closed session of the regional parliament is not recognised by Kiev, dismissed opponents' accusations that he will fix the referendum on Moscow's orders."We guarantee that all aspects of European law will be followed, including security for voters," he said in an interview.
Western countries dismiss the vote as illegal. "The referendum on Sunday will have no legitimacy, no legal effect, it can have no moral effect. It is a piece of political theatre that is being perpetrated at the barrel of a gun," Daniel Baer, the US ambassador to the OSCE, told reporters in Vienna.
Russia has taken territory from its former Soviet neighbours in the past with no serious consequences - in 2008 it invaded Georgia and seized two breakaway regions. But if Putin was hoping for a similarly tepid response this time, he may have misjudged.
In particular, he seems to have alienated Ms Merkel, the Western leader with whom Mr Putin - a German speaker who was once a KGB spy in East Germany - has had the closest relationship.
Ms Merkel was initially more cautious than other Western leaders on the Crimean crisis, but in recent days she has pushed the EU to match US sanctions.
EU action is critical because Europe does 10 times as much trade with Russia as the US, buying most of its gas and oil exports.
The prospect that EU measures could be implemented as soon as Monday has weighed down the Russian economy.
Yet none of that appears to have slowed down Mr Putin, who told officials of the Winter Paralympic Games he is hosting in Sochi that Russia was "not the initiator" of the crisis.
The Russian Defence Ministry said 8,500 troops were taking part in new military exercises near the Ukrainian border, testing artillery and rocket launchers.
It was the second big exercise Moscow has ordered since the crisis began; the first, involving 150,000 troops, began a few days before Russian forces seized Crimea.
In a gesture of support for NATO's eastern members, US F-16 fighter jets landed at Poland's Lask air base today.
Among efforts by the West to isolate Russia politically, the Organization for Economic Co-operation and Development, a 34-member rich nations' club, announced it was suspending membership talks with Russia, under way since 2007.
Moscow has pledged to respond in kind to any Western sanctions. The prime minister of Lithuania said Russia had suspended food product imports through its port of Klaipeda.
But European leaders appear to be calculating that the damage to Russia would be far worse than to Europe.
EU-Russian trade makes up 15% of Russia's economy and just one per cent of Europe's.
Although EU countries depend on Russian gas imports, storage tanks are full after a mild winter season.
Diplomatic lines have been open between Russia and the West throughout the crisis.
US Secretary of State Kerry and Russian Foreign Minister Sergei Lavrov spoke today as they have nearly every day. They are due to meet in London tomorrow.
Russia's top general discussed Ukraine with the chairman of NATO's Military Committee by telephone today, the Interfax news agency said.
The crisis over Crimea began after Mr Yanukovych fled Kiev and pro-European politicians took charge, following three months of demonstrations.
In Strasbourg, the European Court of Human Rights warned Russia and Ukraine that any military action that harmed civilians would violate an international convention signed by both countries.