Government and rebel forces have exchanged dozens of prisoners captured during fighting in Ukraine as part of a cease-fire agreement sealed earlier this month.

The transfer took place outside the main rebel stronghold of Donetsk early Saturday under the watch of international observers.

President Petro Poroshenko says 36 Ukrainian servicemen were released after negotiations. He says another 21 soldiers were freed the day before.

Ukrainian forces handed over 31 rebels detained over the course of the five-month conflict.

Hundreds of prisoners are believed to be held by both sides, although no firm figure has been provided by either.

A cease-fire between separatists and the Ukrainian military took effect last Friday but has already been serially violated. Shortly after the prisoner exchange, a volley of rocket fire was heard from central Donetsk.