By Aleksandar Vasovic and Gabriela Baczynska MARIUPOL/DONETSK Ukraine (Reuters) - Ukrainian forces and pro-Russian rebels clashed just east of the strategic port of Mariupol on Friday, hours before envoys from Ukraine and Russia were expected to announce a ceasefire as the starting point for a wider peace plan. Renewed gunfire and artillery fire were also heard in Donetsk, the rebels' main stronghold in eastern Ukraine, from near the city's airport which remains in government hands. 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 the Russian-annexed Crimea region.