Ukraine's navy said a new temporary Black Sea "humanitarian corridor" had started working on Thursday and that the first ships were expected to use it within days.

Oleh Chalyk, a spokesperson for the Ukrainian navy, told Reuters that the corridor would be used by commercial ships blocked at Ukraine's Black Sea ports and for grain and agricultural products.

"Today a new temporary humanitarian corridor has started to work," Chalyk said by phone.

"The corridor will be very transparent, we will put cameras on the ships and there will be a broadcast to show that this is purely a humanitarian mission and has no military purpose," he said.

The navy said in a separate statement that the risk posed by mines in the Black Sea and the military threat from Russia remained.