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KIEV, Ukraine — The interim central government in Kiev threatened to blockade the eastern city of Slovyansk on Friday, defying warnings from Russia not to confront pro-Russia militants entrenched in towns across eastern Ukraine.

In another affront to Russia, officials in 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 Russia would incite a Russian military incursion that the Kremlin would characterize as a humanitarian or peacekeeping initiative.

In another ominous sign of escalating tensions, the Pentagon said late Friday that Russian fighter jets had made about half a dozen incursions into Ukrainian airspace in the previous 24 hours. Col. Steven Warren, a Pentagon spokesman, said Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, had spoken with his Russian counterpart Thursday, but offered no details.

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Also Friday, a group of foreign military observers traveling under the auspices of the Organization for Security and Cooperation in Europe (OSCE) and their Ukrainian military hosts were detained by pro-Russia 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 Thursday had released an American journalist held for three days, a new set of foreign prisoners.

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.”

Interim Ukrainian leaders said operations to expel pro-Russia militants in eastern cities would continue, even though military action had done little more than harden local sentiments, prompt Russia to stage military exercises on Ukraine’s border and raise concerns about Russia’s next move.

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.

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.

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.

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