Ukraine's interim prime minister has warned the Crimean parliament "no-one in the civilised world" will recognise its referendum on joining Russia.
Arseniy Yatsenyuk and others in the Kiev government have called the vote "unconstitutional" and "illegitimate".
But the referendum has the support of the Russian parliament.
The speaker of the upper house said if the Crimean people vote on 16 March to join Russia then they would "unquestionably back this choice".
The decision by Crimean MPs to seek to join the Russian Federation comes amid international tensions over the presence of pro-Russian troops in the southern Ukrainian peninsula.
Moscow has said it "will not accept the language of sanctions and threats" after the EU and US announced punitive measures against Russia on Thursday.
Meanwhile, Ukraine's Paralympic team has confirmed it will take part in the Sochi Winter Paralympics, which begins later.
The head of the team, Valeriy Sushkevych, said they would participate, but warned: "If something major happens, Ukraine will leave the Games immediately".
Russian energy giant Gazprom meanwhile warned the government in Kiev that it would cut off gas exports if it did not settle a $1.89bn (£1.13bn) debt. In 2009, Gazprom halted supplies to Ukraine in a move that caused shortages across Europe.
Prime Minister Yatsenyuk said on Friday Ukraine was open to talks with Russia as long as it withdrew its troops and abided by international agreements.
In a warning to the "separatist and other traitors of the Ukrainian state", he said: "Any decision of yours is deliberately unlawful and unconstitutional and no-one in the civilised world will recognise the decision of the so-called referendum of the so-called Crimean authorities".
Kiev does not recognise Crimea's pro-Moscow leadership, which was sworn in at an emergency session as pro-Russian forces began to take over strategic sites last week.
Crimea's leadership for its part has branded as "illegitimate" the interim government in Kiev, which was sworn in at the end of February after the ousting of President Viktor Yanukovych.
Crimea's parliament on Thursday said it had already asked President Putin to allow the region to join Russia, and would seek an endorsement from the Crimean people in a referendum on 16 March.
Welcoming a delegation of Crimean MPs to Moscow on Friday, Russia's upper house speaker Valentina Matviyenko said: "Without a doubt, the Crimean parliament, as a legitimate authority, has that right... the sovereign right of the people to determine their future."
Although the majority of people in Crimea are ethnic Russians, the BBC's Mark Lowen in Simferopol says the outcome of a referendum is by no means certain - and may come down to a generational split.
One man said: "I am Russian, but I am not a citizen of the Russian Federation. When I was there several years ago, I understood that we are different people now. I want to feel Crimean."
But his mother-in-law - who remembers when Russia handed over Crimea to Ukraine in 1954 - has a different view.
"I would say yes to joining Russia," she told the BBC. "Crimea has always been Russian territory. We feel separate from Ukraine and I don't have any emotional attachment to this country. Perhaps if the government in Kiev was different then I might vote another way."
For a second consecutive day, 47 military observers from the Organisation for Security and Co-operation in Europe (OSCE) were prevented from crossing into Crimea by armed men guarding the Chungar checkpoint.
The BBC's Bethany Bell says there is "widespread" support at the OSCE's headquarters in Vienna for the need to agree a long-term monitoring mission in Ukraine, but that Russia's delegation insists it has no mandate to deploy.
Russia's foreign ministry accused the European Union of taking an "extremely unconstructive position" after leaders meeting in Brussels agreed to freeze talks on easing travel restrictions to Russians.
Washington also announced visa restrictions against a number of unnamed Ukrainian and Russian officials and individuals.
During an hour-long phone conservation, US President Barack Obama told President Putin that Russia's actions were a violation of Ukrainian sovereignty, the White House said in a statement.
He said there was a solution available that suited all parties, involving talks between Kiev and Moscow, international monitors in Ukraine and Russian forces returning to their bases.
For his part, President Putin said the new authorities in Kiev had imposed "absolutely illegitimate decisions" on the pro-Russian parts of eastern Ukraine, and Moscow "cannot ignore calls for help".
But he also said US-Russian "relations should not be sacrificed due to disagreements over individual, albeit extremely significant, international problems", the Kremlin said.
On Friday, some 65,000 people attended a rally in Moscow expressing solidarity with the pro-Russian authorities in Crimea, police said.
Are you in the affected region? Email us at firstname.lastname@example.org adding 'Ukraine' in the subject heading and including your contact details.