Interim Ukrainian Prime Minister Arseny Yatsenyuk appealed to the United Nations Security Council and Russia’s U.N. ambassador Thursday to prevent war from breaking out between the two former Soviet republics over the political future of the strategic Crimean peninsula.

In Moscow, President Vladimir Putin contended that Russia wasn’t responsible for the turmoil in Crimea and described a Sunday referendum on whether the peninsula should secede from Ukraine as a crisis of a purely "domestic nature."

Yatsenyuk found sympathy for Ukraine’s plight from the Security Council members, except Russia. But the majority’s avowals of support and commitment to Ukraine’s territorial integrity lacked any power to change the reality of Russian military control of the region, where it has long maintained bases.

Yatsenyuk said the Russian intervention in Crimea had "no reasons and no grounds," and he urged Moscow to return its troops to barracks and negotiate an end to the crisis.

"This is absolutely and entirely unacceptable in the 21st century to resolve any kind of conflict with tanks, artillery and boots on the ground," Yatsenyuk told the Security Council, meeting for the sixth time this month on the Crimea standoff.

Crimea’s recently installed pro-Russia leaders have scheduled a referendum Sunday on whether the peninsula, home to Russia’s Black Sea fleet and some of Ukraine’s most lucrative seaside resorts, should declare independence and move to join Russia.

Yatsenyuk switched from English to Russian to appeal directly to Russian U.N. Ambassador Vitaly Churkin, asking him whether his nation really wanted war with Ukraine.

Churkin responded by reminding the council that Ukraine’s interim government failed to fulfill the terms of a Feb. 21 agreement with three European Union countries that was signed by former President Viktor Yanukovich and leading opposition figures, including Yatsenyuk. That agreement, aimed at ending months of violence, called for a unification government and an early presidential election.

Yanukovich and many of his Party of Regions deputies fled after signing the accord, while others defected to the opposition bloc, giving it the majority needed to name a new government. They chose Yatsenyuk to run the country until a presidential election May 25.

Jeffrey Feltman, the U.N. undersecretary general for political affairs, briefed the Security Council on reports that thousands of Russian troops had infiltrated Ukrainian government and military sites and taken control of entry checkpoints at ferry terminals, airports and the main road from the Ukrainian mainland.

"The scheduled referendum has further complicated an already difficult and volatile situation," Feltman said.

The U.N. envoy sent to observe the vote, Assistant Secretary-General Ivan Simonovic, has been barred from entering Crimea, a rebuff that spotlighted the world body’s impotence in the standoff with Russia, which wields veto power on the Security Council.

U.S. Ambassador Samantha Power told the Security Council that 20,000 Russian troops were in Crimea. She called for the referendum to be canceled, arguing that it "cannot be regarded as legitimate, especially against the background of foreign military intervention."

Western nations nonetheless stepped up the pressure on Putin to defuse the conflict or face economic sanctions.

The Russian stock market sent its own note of caution; share values fell Thursday to their lowest level in nearly four years and the ruble continued its nose dive.

The world’s top economic powers appear to be isolating Russia, at least symbolically. The Group of 7 industrialized nations, which admitted Russia as a member in 1998 (making it the Group of 8), has suspended preparations for a June summit in Russia. U.S. and European officials have compiled lists of Russian officials who could be subject to visa bans and asset seizures.

Earlier Thursday, Putin told Russian Paralympics officials in Sochi that his country did not spark the turmoil in Crimea or elsewhere in Ukraine.

"I would like to assure you that Russia did not initiate, it was not an instigator of these difficult circumstances," Putin said.