By Louis Charbonneau
UNITED NATIONS, March 7 (Reuters) - The United Nations voiced concern on Friday about plans by regional authorities in Crimea to hold a referendum on whether to secede from Ukraine and join Russia and warned against decisions being taken "in the heat of the moment."
The east-west confrontation sparked by the overthrow last month of President Viktor Yanukovich after violent protests in Kiev escalated on Thursday when Crimea's parliament, dominated by ethnic Russians, voted to join Russia and called for a March 16 plebiscite on the issue.
"The recent announcement by the authorities in Crimea that they intend to hold a referendum is a worrying and serious development," U.N. spokesman Martin Nesirky told reporters.
"The Secretary-General urges the authorities in Ukraine, including in Crimea, to treat this matter with calm," he said. "It should be noted that referendums usually have clear rules on national constitutional law that should be looked into carefully and dispassionately."
European Union leaders and U.S. President Barack Obama denounced the referendum as illegitimate, saying it would violate Ukraine's constitution. The United Nations did not directly comment on the legality of such a referendum.
"All concerned should think about the implications of any hasty actions or decisions taken in the heat of the moment," Nesirky said. "The Secretary-General cannot emphasize enough the need for peace and stability in the region."
Separately, the U.N. assistant secretary-general for human rights, Ivan Simonovic, plans to visit western and eastern Ukraine soon, including the pro-Russian Crimea region, where another U.N. envoy, Robert Serry, was forced out on Wednesday.
Serry had to abandon his mission to Crimea after he was stopped by armed men and besieged inside a cafe by a hostile crowd shouting "Russia! Russia!" The Dutch diplomat later flew to Istanbul before returning to Kiev.
A group of international observers was prevented from entering Crimea on Thursday by men in military fatigues, Poland said.
Nesirky was asked what the United Nations was doing to ensure that Simonovic does not encounter similar obstacles in his mission to investigate allegations of human rights abuses coming from all sides in the conflict.
He said the United Nations was making clear to all parties that they hope Simonovic will have full access wherever he goes during his eight-day mission in Ukraine.
The new Kiev government says Yanukovich, a Russian ally who was ousted on Feb. 22 after months of protests, was guilty of rights abuses and murder for ordering a violent crackdown on protesters that left more than 80 people dead.
Russia and pro-Russian Crimean authorities have accused the opposition and new government of human rights abuses, including threats and harassment of ethnic Russians and Russian speakers in eastern Ukraine. Western powers and Kiev say there is no evidence that Russian speakers have been threatened. (Reporting by Louis Charbonneau; Editing by Phil Berlowitz)