Fierce fighting has surged in eastern Ukraine as Russian-backed separatists mounted a major offensive to capture a strategic railway hub ahead of a weekend ceasefire.

At least 25 people were killed across the region, officials reported.

Clashes appear to have increased since a peace agreement was sealed yesterday in the Belarusian capital of Minsk by the leaders of Russia, Ukraine, Germany and France.

German chancellor Angela Merkel had cautiously described the ceasefire that takes effect a minute after midnight on Sunday as "a glimmer of hope".

The agreement only appeared to spark a last-minute grab for territory that left at least 25 people dead. The government-held railway town of Debaltseve was on the receiving end of dozens of artillery and rocket salvos in the 24-hour period following the Minsk talks, Ukrainian military officials said.

Military spokesman Andriy Lysenko said 11 soldiers had been killed and 40 wounded over the previous day in eastern Ukraine. Regional authorities loyal to Kiev reported at least seven civilian deaths, while rebels said seven others were killed in artillery attacks on the separatist-held cities of Luhansk and Horlivka.

Reporters observed intense shelling along the main road north of Debaltseve, which remains the town's only land link with the rest of government-controlled territory.

Interfax-Ukraine news agency quoted Petro Mekhed, Ukraine's deputy defence minister, as saying that separatist forces had been given the task of hoisting their flags over Debaltseve, as well as the key port city of Mariupol, by the ceasefire deadline.

Separatist forces have nearly encircled Ukrainian forces in Debaltseve, where all but a few thousand civilians have fled the fighting.

Ukraine says Debaltseve should remain in government control under the terms of a September peace deal. A copy of that agreement leaked to Ukrainian media shows the town lying on the government's side of the line of division agreed by rebel and Ukrainian officials.

But Russian president Vladimir Putin and Ukrainian president Petro Poroshenko appeared to disagree on what the peace deal meant for Debaltseve.

Ukrainian access to the sole highway linking the town to government-held territory looks to have been compromised with the apparent capture of the village of Lohvynove, just north of Debaltseve.

The Donbass Battalion, a unit with Ukraine's National Guard engaged in battles around Lohvynove, said captured combatants had confirmed that Russian troops were actively involved in the battles.

Shells landed as far as Artemivsk, a government-held town 25 miles behind the front line. Reporters saw the body of a child killed after rocket fire hit a nursery there. It was not immediately clear who was responsible for the attack.

Moscow denies providing manpower and weapons to rebel forces, but the sheer quantity of powerful weapons at the separatists' disposal has strained that position.

Russian state news agency RIA-Novosti quoted Kremlin spokesman Dmitry Peskov as saying Moscow would act only as a guarantor in the peace process, and could not affect developments on the ground.

"We simply cannot do this physically, because Russia is not a participant in this conflict," Mr Peskov was quoted as saying.

Elsewhere, by the Azov Sea in south-eastern Ukraine, government troops said they had clawed back a handful of villages. Troops have denied reporters access to those operations, which aim to push rebel forces away from government-held Mariupol.

The ceasefire is to be monitored by the Organisation for Security and Cooperation in Europe's observer mission in Ukraine. OSCE secretary general Lamberto Zannier said in Kiev that he hoped hostilities would be halted by the deadline.

"We would really hope to see a decrease already between now and that moment," he said.

Mr Zannier said combatants would have to do more to enable the OSCE peace-monitoring mission, which uses drone cameras, to properly fulfil its mandate.

"Aerial vehicles have been targeted more than once, monitors have been taken hostage, so we need a change of attitude," he said.

The next step after the ceasefire is to form a sizeable buffer zone between Ukrainian forces and Russia-backed rebels. Each side is to pull heavy weaponry back from the front line, creating a zone roughly 30-85 miles wide, depending on the calibre of the weapons. The withdrawals are to begin on Monday and should be completed in two weeks.

Other thorny political questions, including a degree of autonomy for the disputed eastern regions, are to be settled by the end of the year.