The Organization for Security and Cooperation in Europe (OSCE) says Crimea's planned March 16 referendum on joining Russia is illegal in its current form.
Europe's security and democracy watchdog also says that the organization will not send a mission to observe the poll.
Swiss Foreign Minister Didier Burkhalter, whose country is the current chair of the Vienna-based OSCE, made the announcements in a statement appearing on the organization's website on March 11.
Russia's RIA news agency said March 10 that Ukraine's Crimea region had invited the OSCE to send a mission to observe the March 16 plebiscite. It said the invitation had been issued by the region's pro-Russian parliament.
Meanwhile, U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov spoke on the Ukraine crisis, but their telephone conversation did not yield results.
The call came after the United States over the weekend sent Russia a set of questions which Washington says were aimed to determine what diplomatic steps Moscow was willing to take to resolve the crisis diplomatically.
State Department spokeswoman Jen Psaki said Russia's responses made clear its position had not changed.
"[U.S. State] Secretary [John] Kerry, during [his] call [with Russian Foreign Minister Sergei Lavrov] this morning, made clear that any further escalatory steps will make the window for diplomacy more difficult," Psaki said.
"He also reiterated his willingness to continue to engage with Foreign Minister Lavrov, including this week, but that the environment has to be right and the goal must be to protect the immunity and sovereignty of Ukraine and we didn't see that, obviously, in the responses that we received back."
She said Kerry told Lavrov it is unacceptable for Russian forces to continue to take matters into their own hands in Ukraine.
In Moscow, the Russian Foreign Ministry said Lavrov had told Kerry that any solution to Ukraine's crisis must consider the interests of all Ukrainians and also respect "the rights of residents of Crimea to determine their future."
Both sides said their top diplomats were ready to continue talks.
Preparing For Referendum
The Foreign Ministry separately said that a declaration of independence approved earlier by the pro-Moscow regional parliament in Crimea was "absolutely lawful."
Pro-Russian lawmakers in Crimea, which is now occupied by Russian forces, approved the declaration, with 78 out of the 81 lawmakers present voting in favor.
The March 11 move comes ahead of a referendum on March 16 on whether Crimea should become part of Russia.
It appeared to be aimed at creating a legal framework for the region joining Russia as a sovereign state.
Ukraine's parliament has warned the parliament in Crimea that it faces dissolution unless it cancels the referendum.
Western nations and the new government in Kyiv have said they will not recognize the vote.
Ukraine's acting president Oleksandr Turchynov called the upcoming referendum a "sham" in an exclusive interview with AFP on March 11.
He said, "What they call the referendum will not happen in Crimea but in the offices of the Kremlin."
He also said that Kyiv will not intervene militarily in the separatist peninsula of Crimea, in order to avoid exposing Ukraine's eastern border.
Meanwhile, Ukraine's ousted President Viktor Yanukovych in a statement delivered from the southern Russian city of Rostov-on-Don, reiterated that he is still the country's legitimate leader and vowed to return to Kiyv soon.
It was only the second public appearance by Yanukovych since he fled Ukraine in late February following three months of antigovernment protests. The European Commission has adopted a proposal to extend unilateral trade benefits to Ukraine worth nearly 500 million euros ($693 million) a year.
In Brussels, the European Commission has adopted a proposal to extend unilateral trade benefits to Ukraine worth nearly 500 million euros ($693 million) a year.
EU Commission President Jose Manuel Barroso said the move was "a concrete, tangible" measure of support for Ukraine after the ouster of President Viktor Yanukovych.
Once member states and the European parliament have given their approval, the decision will unilaterally remove or reduce import duties on a wide range of agricultural and other goods.
European Trade Commissioner Karel De Gucht said the tax breaks would run until at least November 1 this year, by which time the European Union expects to have signed a full free-trade agreement with Ukraine.