Ukrainian forces pounded armed separatists to the east of the port of Mariupol on Friday, hours before envoys from Ukraine and Russia were expected to announce a cease-fire as the starting point for a wider peace plan.

Ukraine says its forces are trying to repel a big offensive by the rebels to take Mariupol, a port city of around 500,000 on the Sea of Azov crucial for its steel exports. It stands about halfway between Russia and Crimea.

"Our artillery has come and is being deployed against the rebels," the mayor of Mariupol, Yuri Khotlubey, told Ukraine's 112 TV channel.