MINSK, Feb. 12 - Germany, France, Russia and Ukraine agreed to a deal offering a "glimmer of hope" for an end to conflict in eastern Ukraine, but the United States and NATO said further intense fighting on Thursday ran counter to the spirit of the accord, Reuters reported.

The agreement, announced after more than 16 hours of discussions in the Belarussian capital Minsk, was followed swiftly by allegations from Kiev of a new, mass influx of Russian armor into rebel-held eastern Ukraine.

It calls for a ceasefire between Ukrainian forces and Russian-backed separatists starting Sunday, the withdrawal of heavy weapons from the front line and constitutional reform to give eastern Ukraine more autonomy.

Fighting has intensified in recent days as the rebels try to take control of Debaltseve, a strategic transport hub that would link the two separatist-controlled areas of eastern Ukraine, where elections are contemplated under the accord.

The White House, under pressure from Congress to provide arms to the stretched Ukrainian military, said the deal was "potentially significant" but urged Russia to withdraw soldiers and equipment, and give Ukraine back control over its border.

"The United States is particularly concerned about the escalation of fighting today, which is inconsistent with the spirit of the accord," it said in a statement.

NATO Secretary General Jens Stoltenberg echoed that line and told Norwegian news agency NTB: "Russia must end its support for the separatists and withdraw its forces and military equipment from eastern Ukraine."

Russia denies arming the rebels and sending troops to fight alongside them, despite what Ukraine and its Western allies say is overwhelming evidence. The conflict has killed more than 5,000 people since last April.

Keeping up the pressure on Russia, diplomats said the European Union would go ahead on Monday with a new round of sanctions against 19 Ukrainian separatists and Russians, regardless of the new ceasefire.

The asset freezes and travel bans, the latest in a long series of sanctions by the EU and United States, have piled intense economic pressure on Russia's energy-exporting economy, which has also been hit by a halving of world oil prices since last June.

After an EU summit in Brussels, the leaders of Germany, France and the European Council said wider sanctions were possible if Russia violated the ceasefire agreement.

U.S. officials also said they were not taking sanctions off the table and bluntly warned the separatists against seizing more land before Sunday's ceasefire formally takes effect.

"We are trying to send the message as strongly as we can that any effort to grab more land between now and Saturday night ... will seriously undercut this agreement," a senior U.S. official who spoke on condition of anonymity told reporters in Washington.

The Minsk talks were the culmination of a dramatic initiative by France and Germany following an upsurge in fighting in which the separatists tore through an earlier ceasefire line agreed to last September.

Russian President Vladimir Putin accused Ukraine of prolonging the negotiations, which seemed close to failure at several points.

German Chancellor Angela Merkel said Ukraine President Petro Poroshenko "did everything to achieve the possibility of an end to the bloodshed." She said Putin put pressure on the rebels to agree to the truce "towards the end" of the talks.

"This is a glimmer of hope, no more no less," Merkel told reporters on arriving, straight from the talks in Minsk, at a European Union summit in Brussels. "It is very important that words are followed by actions."

The agreement addressed some of the main stumbling points, including a "demarcation line" between separatists and Ukrainian forces, which the rebels wanted to reflect gains from a recent offensive that shredded an earlier ceasefire deal.

The compromise was that the rebels will withdraw weapons from a line set by the earlier Minsk agreement in September, while the Ukrainians will withdraw from the current front line, creating a 50 km (30 mile)-wide buffer zone.

Ukraine will also get control of its border with Russia, but in consultation with the rebels and only after the regions gain more autonomy under constitutional reform by the end of 2015.

Kiev has made clear, however, that it will not accept independence for the "People's Republics" the rebels have declared.

The ceasefire and heavy weapons pullback would be overseen by the Organization for Security and Cooperation in Europe, a Europe-wide security body. (rt/ez)