Pro-Russian separatists shot down a military helicopter in the eastern Ukrainian city of Sloviansk on Thursday. All 14 people on board died, including a general, Ukraine's acting president confirmed.

"I have just received information that terrorists using Russian anti-aircraft missiles shot down our helicopter near Slaviansk. It had been ferrying servicemen for a change of duty," acting President Oleksander Turchinov told parliament.

An Associated Press reporter in Sloviansk witnessed the helicopter go down and said there was gunshots around the crash site as a Ukrainian air force jet flew overhead.

A Ukrainian Army military jet flies over a shot down Ukrainian Army helicopter outside Sloviansk, Ukraine, Thursday, May 29, 2014.

Sloviansk, a city of 120,000 located about 100 miles from the Russian border, has become the epicenter of fighting in eastern Ukraine over the past few months as armed pro-Russian rebels have seized government buildings and strategically stationed themselves around the city and its outskirts.

Meanwhile, just 70 miles south of Sloviansk, in Donetsk, two pro-Russian groups were in a standoff against each other in an apparent power struggle. As a thunderstorm ominously rolled in through the city, the pro-Russian Vostok Battalion surrounded a regional government building that serves as headquarters for the Donetsk People's Republic and ordered everyone out.

Dozens of armed men from the battalion have taken positions behind trees and around the building's perimeter.

Vostok Battalion besieged the DPR HQ on Thursday in Donetsk, Ukraine.

Despite its looks, the Vostok Battalion said this is not a coup but rather a "police operation" against looters it claims are in the building.

This front loader just pushed aside the barricade of tires and then cleared the road of a truck used as a roadblock. pic.twitter.com/vVVVKbhT0w — Christopher Miller (@ChristopherJM) May 29, 2014

In Donetsk on Monday, rebels who were armed to the teeth seized the airport and ignited a battle against the military, which unleashed airstrikes against the fighters. Two civilians and at least 30 pro-Russian separatists were killed in what is now the largest military assault since the crisis started.

Though the city was eerily calm in the days after the clash — streets were mostly empty as some business shut their doors to ensure safety of workers and customers — another battle seems to be brewing.

Additional reporting by Christopher Miller in Ukraine.