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 troubled east.
Al Jazeera's Kim Vinnell, reporting from the ground on Tuesday, said military aircraft were despatched from Izyum to Kramatorsk to bring the seized airfield there back under the government control.
Russian media was circulating unconfirmed reports of casualties. Russian news agency Itar-Tass later reported, citing witnesses from the scene, that one of four Ukrainian military jets has been shot down.
Meanwhile, Oleksandr Turchynov, Ukraine's acting president said Kramatorsk airfield was regained from pro-Russian armed rebels, Interfax news agency reported.
There were also reports of Ukrainian military vehicles entering the eastern city of Slovyansk.
Dozens of Ukrainian special forces 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 President Oleksandr 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.
"[Russia wants] the whole south and east of Ukraine to be engulfed by fire," Turchynov said.
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 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.
Asked by Al Jazeera's Jonah Hull if he had seen evidence of Russian involvement, Rasmussen said: "We never comment on intelligence, but I think 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 the pro-Russian operations in Slovyansk.
It named him as Igor Strelkov, and said he also coordinated Russian troops in Crimea during the seizure of military facilities there.
Russian officials deny any involvement.