Fighting in eastern Ukraine has claimed more lives overnight, as calls for a ceasefire to be extended past 10pm local time Friday remain unanswered.

Four members of the Ukrainian forces were killed and five wounded when a military post was stormed by rebels near Kramatorsk.

Attacks have marred the week-long ceasefire across the whole of eastern Ukraine, centering on the flashpoint towns of Luhansk and Donetsk.

Early Friday morning rebels released four out of eight international observers kidnapped last month.

The remaining four are being held in Luhansk.

The gesture is being greeted as a sign of willingness to end the conflict which has led to the deaths of more than 400 people.