Ferocious fighting raged in Ukraine on Saturday, threatening a ceasefire deal as Kiev and the U.S. accused Russia of fueling a rebel onslaught to grab territory hours before the truce began.

Kiev-loyal regional police chief Vyacheslav Abroskin said constant artillery bombardments were razing the strategic railway hub of Debaltseve, where Ukrainian forces were clinging on.

The separatist guard at a crossing point between Ukraine and Russia poured scorn on Kiev regaining control despite a new peace deal. "I don't think so, we'll stay right here," he said.

The pro-Russian rebels control around 400 kilometres (250 miles) of Ukraine's border with Russia, and Kiev resuming control was one of the toughest negotiating points at Thursday's Minsk summit that resulted in a peace roadmap for eastern Ukraine.

Russia has "significantly increased" its offensive against Ukraine since a ceasefire was agreed in Minsk this week, Ukrainian president Petro Poroshenko said on Friday.

"Unfortunately after the Minsk agreement, Russia's offensive has significantly increased. We still think that the agreement is in great danger," he added, speaking during a meeting with Hungarian Prime Minister Viktor Orban.

The United States charged Friday that Russia was continuing to deploy heavy weapons in eastern Ukraine, before a ceasefire agreed this week goes into effect.

State Department spokeswoman Jen Psaki said the United States had received reports of heavy weapons being moved into eastern Ukraine from Russia over the past few days, and more apparently on the way.

Fighting raged in Ukraine on Friday as the clock ticked down to a ceasefire that will be a first test of the commitment by Kiev and pro-Russian separatists to a freshly-inked peace plan.

At least 27 civilians and soldiers died in shelling as German Chancellor Angela Merkel warned Russia it risked fresh sanctions if the fighting did not stop.

Russia wanted an immediate ceasefire in Ukraine and the Sunday morning deadline was agreed in talks in Minsk under pressure from rebel leaders, President Vladimir Putin's spokesman was quoted as saying Friday.

Russia "supported an immediate ceasefire," Putin's spokesman Dmitry Peskov was quoted as saying by the Kommersant business daily. 

French President Francois Hollande said Thursday the conditions were "still not right" to go ahead with the sale of two Mistral warships to Russia despite Moscow having backed a new Ukraine peace plan.

Hollande suspended the 1.2 billion euros ($1.5 billion) sale last year as the Ukraine crisis deepened, fearing it would boost Russia's military capabilities and put Paris in hot water with its allies.

The United States cautiously welcomed a brokered ceasefire in the 10-month war between Ukraine and pro-Moscow rebels Thursday, but insisted the deal must be implemented and honored.

"The agreement represents a potentially significant step toward a peaceful resolution of the conflict and the restoration of Ukraine's sovereignty," the White House said in a statement.

The Ukraine ceasefire deal agreed Thursday appears little more than a sticking plaster that will allow Kiev and Ukraine rebels to wind down the fighting but leaves thorny questions on resolving the conflict unanswered, analysts said.

Some 16 hours of talks in the Belarussian capital Minsk -- which saw one reporter collapse with high blood pressure as negotiations continued through Wednesday night -- produced an agreement on a ceasefire to take effect from Sunday and the withdrawal of troops.

Western European leaders voiced cautious optimism after hammering out a Ukraine peace plan in Minsk on Thursday, hailed as a great victory by pro-Russian rebels but met with skepticism by war-weary civilians.

German Chancellor Angela Merkel, who was a driving force behind the negotiations, along with French President Francois Hollande, said she had "no illusions" and that "big hurdles" remained in ending the 10-month conflict.