Heavy artillery fire roared Saturday in eastern Ukraine as fighting between Ukrainian forces and Russia-backed separatists continued hours before a cease-fire was to take effect in the conflict that has killed more than 5,300 people.

Associated Press reporters saw the artillery barrage near the town of Svitlodarsk as well as considerable movement of Ukrainian forces' armored vehicles and rocket launchers along the road.

Svitlodarsk is about 20 kilometers (12 miles) north of Debaltseve, a strategically important railroad junction city where Ukrainian forces have been under siege by rebels. Eduard Basurin, a rebel spokesman, was quoted by the RIA Novosti news agency as saying that the fighters would not allow the Ukrainian forces to escape the city but would be offered the opportunity to surrender.

Under an agreement reached Thursday, the warring sides are to cease firing at midnight (2200 GMT). However, a previous cease-fire called in September failed to take hold and after fighting escalated sharply in January, expectations for the new agreement are clouded.

Officials in the port city of Mariupol meanwhile said an array of artillery attacks hit areas near the city during the morning. There was no immediate information on casualties.

Mariupol is on the Azov Sea and concerns are strong that Russian-backed separatists aim to seize it as a step toward establishing a corridor between mainland Russia and the Crimean Peninsula that Russia annexed 11 months ago.

Also Saturday, the U.S. ambassador to Ukraine, Geoffrey Pyatt, posted on Twitter what he said were satellite photos showing Russian artillery systems near the town of Lomuvatka, about 20 kilometers northeast of Debaltseve. The images could not immediately be verified.

Russia has flatly denied repeated Western claims that it has sent troops and equipment to the eastern Ukraine rebels.

The fighting started in April after armed separatists took control of towns and official buildings in the Dometsk and Luhansk regions of eastern Ukraine. The seizures began after Russia-friendly president Viktor Yanukovych was driven from power in the wake of months of protests in the capital, Kiev. The separatists claim the new Ukrainian authorities are fascist-inspired and aim to suppress the heavily ethnic Russian population in the east.