Pro-Russian volunteers gather in a square next to the Council of Ministers of Crimea's building, in Simferopol. The US and Russia failed to resolve a Cold-War-style standoff sparked by Moscow's military intervention in Crimea.

Ukrainian authorities apparently blocked all international flights into the embattled eastern region of the country Tuesday, as the interior minister said four government soldiers and perhaps more than 30 pro-Russian rebels had been killed in clashes the previous day near the city of Slovyansk.Authorities also canceled at least 20 flights from Kharkiv, eastern Ukraine's largest city, and Donetsk, a regional capital, as they girded for violence ahead of a holiday Friday commemorating victory in World War II. By midafternoon, all air traffic within Ukraine appeared to have ceased.On Monday, Ukraine's acting president, Oleksandr Turchinov, said vehicle checkpoints had been established at strategic points around the capital of Kiev as a precaution against attacks.In a posting on Facebook, the interior minister, Arsen Avakov, accused the pro-Russian separatists of using civilians as human shields. "The Ukrainian military can't fire on civilians," Avakov said. "That is how they are restricted. And this restriction is used by our enemies. The enemy hides behind people and fires from there."The developments coincided with mounting Western concern that the fighting in the east could spiral out of control and lead to direct intervention by Russia, which has deployed tens of thousands of troops on its side of the border, supposedly for training exercises.In a television interview in Paris, President Francois Hollande of France said that if an election planned by the Kiev authorities for May 25 was not held, "there would be chaos and the risk of civil war."In Vienna, around 30 foreign ministers - including those of Ukraine and Russia - were meeting Tuesday under the auspices of the Council of Europe to debate the crisis.The British foreign secretary, William Hague, suggested at the meeting that Russia is feeding the conflict in Ukraine to undermine the presidential election scheduled for May 25. He told reporters, according to The Associated Press, "Russia is clearly intent on preventing or disrupting those elections." He said the foreign ministers at the meeting supported holding the vote without any outside interference.But Russia's foreign minister, Sergei Lavrov, said at the same meeting that it would be "unusual" to hold a presidential election in Ukraine while the government was using military force across a broad region of the country, according to Reuters.Late on Monday, the German Foreign Ministry in Berlin toughened its travel advisory for Ukraine, recommending that German citizens leave eastern and southern Ukraine. On Sunday, the ministry was strongly advising Germans against entering those areas.Journalists were expressly warned against traveling to the south and east. "In the light of the most recent developments one must assume that media representatives run a particular risk of being blocked or detained by separatist forces," the Foreign Ministry said.Germany spent last week working for the release of four military observers who were captured by pro-Russian militants in Slovyansk. The four men and the other seven people held with their German-led military observer team were freed over the weekend, and Germany has since said it would not send more military observers to Ukraine.Foreign Minister Frank-Walter Steinmeier suggested in a television interview Monday that events were now beyond the control of the authorities in Kiev and Moscow."I'm convinced that we are struggling against a situation that has taken on a dynamic of its own," Steinmeier said. "There are groups in eastern Ukraine that are not listening" to the central governments in Ukraine and Russia, he said.In a separate interview with four European newspapers, Steinmeier said recent violence in the Ukrainian city of Odessa had "shown us that we are just a few steps away from a military confrontation." Conflict has reached a point that "a short time ago we would have considered impossible," he added.Navi Pillay, the top U.N. human rights official, also sounded the alarm, urging "all sides to make a much greater effort to find a peaceful resolution to the current crisis, especially in the various towns in eastern and southern Ukraine that have been racked by increasingly violent confrontations."

© 2014, The New York Times News Service

"Armed opposition groups must stop all illegal actions, including detaining people and seizing public buildings in violation of Ukraine's laws and Constitution. These organized and well-armed groups should lay down their weapons, free arbitrarily detained persons, and vacate occupied public and administrative buildings," she said in a statement.