The Ukrainian government and pro-Russia rebels have signed a truce deal to end almost five months of fighting.
The two sides, meeting in the Belarusian capital Minsk, agreed to stop firing at 15:00 GMT.
President Petro Poroshenko said he would do "everything possible" to end the bloodshed. The rebels said the truce had not changed their policy of advocating separation from Ukraine.
The move prompted a military operation by Ukrainian forces to retake the cities.
The rebels, who had largely been pushed back towards their strongholds of Donetsk and Luhansk, made new advances in recent days.
Fighting was continuing on Friday around Mariupol, a coastal city about 110km (70 miles) south of Donetsk.
The BBC's Fergal Keane in Mariupol tweeted that pro-Russia forces seemed to be hitting Ukrainian positions 4km outside the city.
Large plumes of smoke could be seen as Ukrainian artillery fired back, he says. Ukrainian fighter jets also hit rebel positions.
The West accuses Russia of sending arms and troops to back the rebels in eastern Ukraine - allegations denied by Russia.