Kiev accused Russia on Friday of sending more tanks and troops into eastern Ukraine and said they were heading towards the rebel-held town of Novoazovsk on the southern coast, expanding their presence on what it fears could be the next battlefront.
Russia did not immediately respond to the accusation which, if confirmed, would go further to kill off a European-brokered truce that was met by relentless rebel advances after it came into force on Sunday. Moscow has always denied accusations in the past that its forces are fighting in Ukraine.
Nevertheless, there were signs that the rebels may be prepared to halt their advance, having achieved their main military objective this week by seizing the strategic town of Debaltseve in defiance of the ceasefire.
Reuters journalists in the battle zone, including Debaltseve itself now firmly in rebel hands, said it was quieter than it had been for days. Kiev and the rebels both announced a new agreement, signed by Ukrainian generals and separatist leaders, to begin withdrawing heavy weapons from the front line, one of the main requirements of the truce deal.
Kiev's biggest worry is that rebels will continue their advance to threaten Mariupol, a highly strategic port of 500,000 people that is the biggest city still under government control in the two rebellious eastern provinces. Novoazovsk, where Kiev said Russia was reinforcing, lies 40 km (25 miles) to the east along the coast near the Russian border.
"In recent days, despite the Minsk (ceasefire) agreement, military equipment and ammunition have been sighted crossing from Russia into Ukraine," military spokesman Andriy Lysenko said. He said more than 20 Russian tanks, 10 missile systems and busloads of troops had crossed the frontier.
The United States, which is considering tightening sanctions against Russia and arming Kiev, also says it has sighted Russian reinforcements. The State Department said Russian support for the rebels was undermining international diplomacy and would bring "costs" on Moscow.
Western nations have held out hope they can revive a peace deal brokered by France and Germany in the Belarussian capital Minsk on Feb. 12, even though the rebels ignored it to seize Debaltseve, a town on a strategic railway hub, inflicting one of the worst defeats of the 10-month-old war on Kiev.
"We are more convinced than ever that they must be applied - all the agreements, nothing but the agreements," French President Francois Hollande said in Paris alongside German Chancellor Angela Merkel on Friday.
"These are ours now"
In Debaltseve, where Kiev was forced to withdraw thousands of besieged troops this week, the black, blue and red flag of the rebels' self-proclaimed Donetsk People's Republic flew over the town hall. Artillery could be heard in the distance.
Walls were pock-marked by bullets or had huge holes blown in them. Burnt-out hulks of tanks and military trucks lay abandoned on the road. Heavily armed rebels and tanks patrolled the streets on the outskirts, where bridges had been blown up and the railway appeared to have suffered major damage.
A separatist fighter picked out ammunition left by fleeing government troops from the rubble.
"These are ours now, and we'll use them against them. We have to expand," said the rebel who refused to give his name, straightening up while a dozen others continued their search for arms and ammunition.
The town held around 27,000 people before the fighting. Most fled during the battle, but thousands remained trapped inside, sheltering in cellars during days of relentless bombardment. Several hundred lined up for food in the centre of town. Many were weeping as they stood in the queue.
"We're waiting. They should bring dry meals in blue packaging since the pasta is already finishing," said Yulia, 28, a teacher, holding her 10-year-old niece by the hand.
Dark mood at uprising anniversary
In the capital Kiev, the military setbacks darkened the mood as crowds gathered for the first anniversary of an uprising that toppled the Moscow-leaning president Viktor Yanukovich and culminated in war.
Ukraine's President Petro Poroshenko accused a top aide to Russia's Vladimir Putin of being behind the sniper killings of 100 Ukrainian protesters on the streets of Kiev during the uprising a year ago.
"Just a few days ago, the head of state security told me that special forces operatives gave evidence that the Russian presidential aide Vladislav Surkov led the organisation of groups of foreign snipers on the Maidan," Poroshenko told some of the relatives of those killed, according to his website.
Russia's Foreign Ministry called the accusation "madness".
Crowds headed to observe the anniversary at Independence Square in the heart of the capital, known locally as "the Maidan" and revered by Ukrainians as platform of a revolution that turned the country away from Russia and towards Europe.
"We have shown the world, we have shown everyone that we can stand against the regime without weapons, that we are free people and we will fight till the end, till victory," said Mykola Tokar who travelled from the Lviv region in the west to attend the ceremonies in Kiev.
Some people wore combat fatigues showing allegiance to pro-government militias that have fought alongside Kiev's troops against pro-Russian separatists. The mood was sombre.
More than 5,600 people have been killed in fighting since mid-April last year, soon after Russia annexed Crimea from Ukraine following Yanukovich's overthrow.