Russia and Ukraine: Why tensions between them are so high
There has been growing hostility between Ukraine and Russia over access to the Black Sea and the Sea of Azov off the Crimean peninsula - annexed by Russia in 2014.
On Sunday Russia captures three of Ukraine's naval vessels and 23 crew members after accusing Ukraine of illegally entering its waters. Ukraine said it was a Russian "act of aggression".
Now Ukraine's parliament is to decide whether to bring in martial law, after Sunday's capture of three of its naval vessels and 23 crew members by Russia.
But how did we get here?
The BBC's Jonathan Marcus explains.