Ukrainian President Petro Poroshenko said the continuing bombardment of civilians in eastern Ukraine by pro-Russian separatist rebels was already undermining the peace plan reached in Minsk on Thursday.
At least 28 civilians and soldiers were reported killed in Friday's upsurge in fighting.
"Unfortunately after the Minsk agreement, Russia's offensive has significantly increased. We still think that the agreement is in great danger," Poroshenko said during a meeting with Hungarian Prime Minister Viktor Orban.
"After what we concluded in Minsk, these are not only attacks against civilians but also against the Minsk agreement."
The ceasefire, due to take effect from midnight on Saturday (Sunday AEDT), will be the first test of the commitment by Kiev and pro-Russian separatists to the freshly-inked peace plan.
But with separatists fighting to conquer more territory ahead of the truce and Kiev forces digging in, there are fears over whether anyone will observe the truce, considered vital to the success of the peace roadmap.
The United States said it believed Russia was continuing to deploy heavy weapons ahead of the ceasefire.
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.
"This is clearly not in the spirit of this week's agreement," Psaki told reporters.
She said the Russian military had deployed large amounts of artillery and multiple rocket launcher systems and was using them to shell Ukrainian positions.
Friday's fresh fighting came after rebels and Kiev agreed to the wide-ranging plan on Thursday following marathon talks in the Belarussian between the leaders of Russia, Ukraine, France and Germany.
German Chancellor Angela Merkel has warned Russia that the EU, which has already slapped Moscow with sanctions over the crisis, is not ruling out further measures if the truce fails.
The fragile agreement was seen as the best hope of ending the conflict, which has killed at least 5,480 people and ratcheted East-West tensions to highs not seen since the Cold War, but scepticism remains high after the collapse of a similar previous peace plan.
The new Minsk agreement is broadly similar to an earlier failed deal in September, except that the new heavy weapons-free zone will be 50 to 140km-wide, depending on the range of the weapon, double the width of the buffer zone agreed in September.
Kiev will also begin retaking control over the approximately 400km stretch of Russia's border with rebel-held Ukraine, but only after local elections are held.
Separatist-held territories will be granted a degree of autonomy to be established through talks.
