Ukraine says Russia has closed off part of Black Sea

The Ukrainian border guards service said on Saturday Russia had closed off the northwestern part of the Black Sea to navigation.

"The presence of ships and ships in this zone will be regarded as a terrorist threat," it said in a statement.

On Friday, Ukraine said Russia had hit two commercial ships near the southern Ukrainian port of Odessa, one day after Russia began a full-scale invasion of Ukraine.

Under the 1936 Montreux Convention, Turkey has control over the passage of vessels between the Mediterranean and the Black Sea, making it an essential player in the current conflict between Ukraine and Russia. 

Here's more on the convention and its implications for the Russia-Ukraine conflict:

Turkey's power over the Black Sea explained