Ukraine's Acting President, Oleksandr Turchynov, said an airfield in the eastern town of Kramatorsk had been taken back from pro-Russian separatists, Reuters reported Tuesday.
The statement from Turchynov's office marks the first news of successful action by Ukraine against pro-Russian separatists, who have occupied a number of government building across the east of Ukraine.
It came after Ukraine's ambassador insisted that Kiev was ready to respond to Russia with full force. Speaking to CNBC earlier on Tuesday, Olexander Scherba, ambassador-at-large of Ukraine, said that soldiers had been deployed to the east of the country.
"There are special forces deployed on the ground in Ukraine's east but I'm not authorized to comment (any more) on that," Scherba said. "We are ready to respond with all forces that we have and we will do that."
Russia said it was deeply concerned on Tuesday over reports of casualties in eastern Ukraine, where Kiev has launched an operation against pro-Russian separatists.
"The reports we are getting cause deep concern. To all appearances, events are beginning to develop under the worst case scenario," Konstantin Dolgov, the Russian foreign ministry's human rights representative, was quoted by state news agency RIA as saying.
A deadline for the separatists to give up their occupation of government buildings across east Ukraine was ignored on Monday, but despite threats from Kiev, military action against the rebels was slow to materialize.
On Tuesday afternoon, Reuters reported that Ukraine had sent airborne troops to Kramatorsk, and that a similar operation was underway in the eastern town of Slaviansk, according to the state security service.