A screen grab of one video that appears to show a helicopter going down.
A screen grab of one video that appears to show a helicopter going down. (YouTube)

On Friday, Ukraine's army launched its first major assault on a rebel stronghold, with the state's military attempting to retake Slovyansk, a region currently controlled by pro-Russian militia.

The assault has seen casualties on both sides, and Ukraine's Defense Ministry said that two Mi-24 attack helicopters were shot down and two crew members killed.

A number of amateur videos posted online appear to show the helicopters being attacked. One video, later shared by the pro-Kiev Euromaidan PR account and picked up by the United Kingdom's Channel 4, appears to show a helicopter going down after being shot. The video has been independently verified by Storyful.

Other footage, which appears to have been shot and uploaded to YouTube by a pro-Russian activist, seems to show a helicopter being shot at over a militia checkpoint outside Sloyvansk. 

And one more amateur video appears to show a crowd cheering in Russian as they see a large explosion in the distance, which the person who uploaded the video says is a Ukrainian helicopter crashing.

While it is difficult to conclusively confirm the authenticity of these videos, a representative of Euromaidan PR said they believed all three were authentic. They are just a handful of the many more that have been uploaded to YouTube over the past 12 hours.

The downing of multiple helicopters is being taken as proof that the pro-Russian militias are at least being armed by Russia. The Ukrainian Security Service is saying that at least one helicopter was shot down by a surface-to-air missile, which led Sweden's Foreign Minister Carl Bildt to tweet: