Pro-Russian activists at a checkpoint outside Slovyansk, Ukraine. Action by Ukrainian forces appeared to be centered on the city of Slovyansk, a militant stronghold.

The interim central government in Kiev threatened to blockade the eastern city of Slovyansk on Friday, defying warnings from Moscow not to confront pro-Russian militants entrenched in towns across eastern Ukraine. (Also Read: R ussian aircraft entered Ukraine airspace: Pentagon In another affront to the Kremlin, Kiev also warned that any Russian troops crossing the border on maneuvers would be treated as an invasion.The declarations reflected heightened worries that the government's efforts to move against forces aligned with Moscow would incite a Russian military incursion that the Kremlin would characterise as a humanitarian or peacekeeping initiative. (Also Read: Window closing for Russia to change course in Ukraine: John Kerry In another ominous sign of escalating tensions, the Pentagon said Friday evening that Russian fighter jets had made about half a dozen incursions into Ukrainian airspace over the previous 24 hours.Col. Steven H. Warren, a Pentagon spokesman, said that Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, had spoken with his Russian counterpart Thursday, but offered no details.Also Friday, a group of foreign military observers traveling under the auspices of the Organisation for Security and Cooperation in Europe, along with their Ukrainian military hosts, were detained by pro-Russian separatists in Slovyansk, the separatists and the Ukrainian government said.The government said seven foreign military observers and five members of the Ukrainian military had been seized. The detention appeared to give the rebels, who on Thursday had released an American journalist held for three days, a new set of foreign prisoners. (Also Read: US warns Russia against 'expensive mistake' in Ukraine Unconfirmed early reports had identified the detainees as monitors from the security organization, but the organization's officials said all of their permanent staff members and monitors had been accounted for. Tatyana Baeva, an organization spokeswoman in Vienna, said the reports evidently referred to a separate German-led group of unarmed military inspectors.The German Embassy in Kiev referred questions to the German Defense Ministry spokesman in Berlin, who could not be reached.The self-declared mayor of Slovyansk, Vyachislav Ponomaryov, was quoted by Russia's Interfax News Agency as saying that rebels were trying to verify the identities of the detainees, who had been on a bus that he said was carrying ammunition.Reached by phone Friday night, his spokeswoman escalated the language, saying the observers had been detained "on suspicion of being spies." (Also Read: Ukraine PM cuts short Rome trip 'because of the situation' She cut short the call before providing details.Interim Ukrainian leaders said operations to expel pro-Russian militants in eastern cities would continue, even though military action so far had done little more than harden local sentiments, prompt Russia to stage military exercises on Ukraine's border and raise concerns about Moscow's next move."Attempts at military conflict in Ukraine will lead to a military conflict in Europe," Ukraine's acting prime minister, Arseniy P Yatsenyuk, told the interim Cabinet, according to remarks broadcast live and posted on the government's website. "The world has not yet forgotten World War II, but Russia already wants to start World War III."The acting head of Ukraine's presidential administration, Serhiy Pashynskyi, said the operation to dislodge what he called terrorists was continuing in and around Slovyansk and would now focus on "totally blockading" the city to prevent militants from getting reinforcements and supplies.He also claimed that Russian military movements overnight at four locations on the Russian side of the border had involved "400 tanks, armored vehicles and rocket launchers."Near Slovyansk, a Ukrainian infantry unit patrolled in the farmland northwest of the city, a day after Ukraine sought to dislodge pro-Russian forces from checkpoints.At a checkpoint on the road just outside of Izyum, northwest of Slovyansk, a combined force of soldiers and Interior Ministry troops were equipped with rifles and machine guns, backed with armored vehicles and at least one transport helicopter."We have a good commander, and we believe him," said Sergei, a major with Interior Ministry troops who gave only his first name. "But this will all be decided up above," he added. "The politicians should resolve it."In nearby Kramatorsk, a Ukrainian military transport helicopter and a civilian An-2 aircraft were destroyed by mysterious fires on the airfield that caused both aircraft to explode. The cause of the blazes was unclear. Officials gave conflicting explanations, including a helicopter being struck at takeoff by sniper fire, an incoming rocket and a mechanical problem.The scene at the airfield, however, was calm, with the pro-Russian and Ukrainian troops separated by a short distance and displaying no overt signs of hostilities.On Facebook, Interior Minister Arsen Avakov said Ukraine's military operations in the east - known as "ATO," meaning anti-terrorist operation - had not been suspended, despite local news reports to the contrary."The ATO goes on," he said. "The terrorists should be on their guard round the clock. Civilians have nothing to fear."In Slovyansk, the city the Kiev government said was under blockade, residents reacted bitterly."What, they will allow no food in, no products?" asked Natalya Ivanyuk, a retired teacher standing with pro-Russian men near a barricade guarded by armed men. "It is like Leningrad. Is it the Second World War?""Blockade!" she added, with disgust. She sharply criticized the interim government in Kiev. "No one comes here and asks us what we want," she said. "Instead they send tanks."At the edges of the city, rebels and work brigades alike could be seen fortifying older positions and building new ones. At one partly erected bunker, beside a canal and a short bridge over a canal, women worked beside men until almost sunset, filling and hauling sandbags.The heightened tensions and continued actions have buried already faint hopes that a deal reached April 17 in Geneva by diplomats from the European Union, Russia, Ukraine and the United States might calm a crisis, stirring fears of a wider conflict over Ukraine, a nation of 46 million on a volatile fault line between Europe and Russia.The threat of intensified Western sanctions was underscored Friday when the rating agency Standard & Poor's downgraded its assessment of Russia."In our view, the tense geopolitical situation between Russia and Ukraine could see additional significant outflows of both foreign and domestic capital from the Russian economy and hence further undermine already weakening growth prospects," the agency wrote.In his most detailed accusation of Russian interference to date, Secretary of State John Kerry said late Thursday that U.S. intelligence services had concluded that Russia's military intelligence services and "special operators" were "playing an active role in destabilizing eastern Ukraine with personnel, weapons, operational planning and coordination."

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On Friday, Kerry's Russian counterpart, Sergey V. Lavrov, hit back, accusing Washington of seeking only to further its interests in Ukraine."The West wants to take control of Ukraine while exclusively putting its geopolitical interests, not the interests of the Ukrainian people, at the forefront," he said at a conference of young diplomats from former Soviet republics. "This is not our method. We will not blackmail, we will not threaten, we are all polite people."