After a firefight near the Russian border in the Ukrainian city of Kramatorsk -- pro-Russian militants seize the police headquarters.

Some 20 men, masked and carrying automatic weapons, shot at the building as they approached.

Some protested the takeover, calling for unity in Ukraine, but were quickly silenced by crowds of pro-Russia supporters.

This comes as Washington announced that U.S. Vice President Joe Biden will travel to Kiev on April 22.

Kiev said it would prepare troops to tackle what it calls an "act of aggression by Russia."