Toward the rebel stronghold of Donetsk
Russia insists the ceasefire outlined by the Minsk protocol is the only way out of the conflict. It denies sending troops and tanks into Ukraine.
"(The collapse of the ceasefire) must not be allowed ... It would be catastrophic for the situation in Ukraine," said Russian Foreign Ministry spokesman Alexander Lukashevich.
The Organisation for Security and Cooperation in Europe, which is monitoring the ceasefire, has reported columns of unmarked soldiers and tanks in Ukraine, traveling away from the Russian border.
A Reuters reporter saw a 50-vehicle column traveling toward the rebel stronghold of Donetsk in eastern Ukraine on Tuesday armed with rocket launchers and artillery guns.
Fresh shelling rocked Donetsk. It was unclear who was shooting or whether it came from the city's airport, where Ukrainian troops and rebels are fighting for control despite the truce.
Kiev, which has boosted its defenses in eastern Ukraine in case of a new offensive, says the reports of the columns back up its accusations that Russia is sending reinforcements to the rebels. A military spokesman said four Ukrainian servicemen had been killed in the past 24 hours.