The fighting between pro-Russian separatists and Kiev's forces continues to escalate in eastern Ukraine as the fragile ceasefire agreement is set to come into effect.

Ukraine's President Petro Poroshenko has warned the ceasefire deal, brokered by France and Germany in Minsk, is in "great danger" if the violence does not stop.

Before Sunday midnight – when the truce comes into force, the rival sides have agreed to pull back heavy weapons from the conflict afflicted regions in eastern Ukraine.

Heavy artillery is being reported in the Donetsk region with tanks being mobilised to step up the battle. Both the rebels - who are accused of being backed by Moscow - and the Ukrainian troops blame each other for the escalation.

"Today, having agreement on a ceasefire from 0000, February 15, Russian-backed militants and the Russian armed forces have been ordered to raise the Russian flag over Debaltseve," Ukraine's Deputy Defence Minister Petro Mekhed told a ministers' briefing over the worsening crisis.

Debaltseve was one of the sticky points during the marathon talks between leaders of Germany, France, Russia, and Ukraine in the Belarusian capital which culminated in the ceasefire agreement.

The Ukrainian deputy defence minister continued: "However, while there is a fierce fighting for Debaltseve, which has been attacked three times as of this morning, Mariupol is still under the control of the Ukrainian armed forces," local reports say.

"According to the latest information, there are more than 10,000 soldiers of the Russian armed forces on the territory of Ukraine."

Meanwhile, the US has blamed Russia for the latest rise in violence saying Moscow does not uphold the "spirit" of the ceasefire terms.

US State Department spokesperson Jen Psaki said Russia has authorised the deployment of "large amount of artillery and multiple rocket launcher systems" in the restive areas.

"This is clearly not in the spirit of this week's agreement."

Psaki went on to add that Washington has intelligence Moscow is poised to dispatch further lethal aid for the rebels.