Ukraine's acting President Oleksandr Turchynov says government forces have captured the airport in the eastern town of Kramatorsk from a pro-Russian group.
Turchynov told parliament the goal of the security operation in Kramatorsk, and another operation in Slovyansk, was "to protect citizens from terrorists who are trying to tear this country apart."
There was shooting during the April 15 siege of the airport, and an unclear number of casualties has been reported.
Russian media has reported that between four and 15 people may have been killed.
Russia's Rossia-24 television channel showed footage of Ukrainian warplanes flying low over the Kramatorsk airport, and helicopters hovering over the area.
Ukrainian forces supported by armored vehicles have also entered the eastern city of Slovyansk, some 15 kilometers south of Kramatorsk.
Reports said some 500 Ukrainian troops backed by 20 armored vehicles and tanks entered Slovyansk in the early evening of April 15.
Ukrainian General Vasyl Krutov said pro-Russian groups occupying government buildings in eastern Ukrainian cities and towns "must be warned that if they do not lay down their arms, they will be destroyed."
Shortly after the Kramatorsk fighting subsided, Russia's Foreign Ministry said it was worried that the "military" campaign under way in Ukraine could lead to "civil war."
Western nations say Russia has massed some 35,000 to 40,000 troops along the eastern Ukrainian border.
Moscow says the troops are taking part in extended military training exercises.
But Western nations have rejected that explanation, saying the Russian force is there to intimidate Ukraine's new government.
Western nations also say they are concerned Russian forces could cross the border to come to the aid of pro-Russian groups.
Russia sent forces into Ukraine's Crimea and annexed the peninsula in March.
But Moscow has denied Ukrainian and Western allegations that Russia has also sent operatives into Ukraine's east to prepare for a possible military intervention.
With reporting by Interfax, Rossiya 24 and AP