Unmarked armed convoys traveling
Moscow and Kiev accused each other on Thursday of violating a ceasefire and the United States warned Russia the West might punish it further for its "military escalation" of the Ukraine crisis.
Ukraine accused Russia of sending soldiers and weapons to help separatist rebels in eastern Ukraine launch a new offensive in a conflict that has killed more than 4,000 people. Russia warned Kiev that any resumption of hostilities against the separatists would be catastrophic for Ukraine.
Increasing violence, truce violations and reports of unmarked armed convoys traveling from the direction of the Russian border have aroused fears that a shaky Sept. 5 truce could collapse.
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.