Ukraine has launched an "anti-terrorist operation" against pro-Russian activists in eastern cities of Kramatorsk and Slovyansk as part of a wider efforts to crack down on separatism in the country's east.
Al Jazeera's Kim Vinnell, reporting from Kramatorsk on Tuesday, said military aircraft were sent from Izyum to bring Kramatorsk's airfield back under government control.
Itar-Tass, the Russian news agency, cited witnesses that one of four Ukrainian military jets has been shot down. That was not confirmed by Ukrainian sources.
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Meanwhile, Oleksandr Turchynov, Ukraine's acting president, was reported by Interfax as saying Ukraine had taken Kramatorsk airfield from pro-Russian rebels.
There were also reports of Ukrainian military vehicles entering the eastern city of Slovyansk.
Dozens of Ukrainian commandos and paratroopers positioned themselves on Monday night in Izyum, on a road leading to Slovyansk, backed by dozens of armoured personnel carriers, seven buses and a helicopter.
The developments came a day after a deadline set by Ukraine's acting Turchynov, calling on pro-Russian separatists to lay down arms and leave occupied government offices, expired on Monday 0600 GMT.
The separatists, many of them armed, continued to occupy government, police and other administrative buildings in at least nine cities in the country's Russian-speaking east of the country, demanding broader autonomy and closer ties with Russia.
On Tuesday, Turchynov announced the start of the "anti-terrorist operation", but gave few details, saying only that it would be conducted in a "responsible and balanced" manner.
He said that Russia wanted "the whole south and east of Ukraine to be engulfed by fire".
He said the government operation aimed to "defend the citizens of Ukraine, to stop terror, stop crime and stop attempts to tear our country into pieces".
For its part, Russia strongly warned Ukraine against using force against the pro-Russian protesters, saying that could prompt the country to walk out of Thursday's international conference on Ukraine in Geneva, Swtzerland.
"You can't send in tanks and at the same time hold talks. The use of force would sabotage the opportunity offered by the four-party negotiations in Geneva," Sergey Lavrov, Russia's foreign minister, said on Tuesday.
But Ukraine found backing from Anders Fogh Rasmussen, NATO's secretary-general, who said in Luxembourg, where NATO ambassadors are due to meet on Wednesday, that Russia was meddling in the crisis in eastern Ukraine.
"From what is visible, it is very clear that Russia's hand is deeply engaged in this."
Ukraine's security services on Tuesday identified a man it said was a Russian foreign intelligence agent who was running pro-Russian operations in Slovyansk.
It named him as Igor Strelkov, and said he also coordinated Russian troops in Crimea. Russian officials deny any involvement.