By Thomas Grove and Natalia Zinets
MOSCOW/KIEV (Reuters) - Moscow and Kiev traded accusations on Thursday of failing to observe a ceasefire agreement and Russia warned a resumption of hostilities against pro-Russian separatists in the east would be catastrophic for Ukraine.
Ukraine accused Russia of sending soldiers and weapons to help rebels launch a new offensive in a conflict that has killed more than 4,000 people.
Increasing violence, ceasefire violations and reports of unmarked armed convoys traveling from the direction of the Russian border have boosted fears that a shaky Sept. 5 truce could collapse.
Moscow denies accusations it sent in troops and tanks in the past few days and says the ceasefire as outlined by the Minsk protocol is the only way out of the conflict.
"(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.
European rights and security watchdog, the Organisation for Security and Cooperation in Europe, which is monitoring the ceasefire, has reported several columns of unmarked soldiers and tanks 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 though 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 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.
Zoryan Shkiryak, an aid to Ukraine's interior minister warned at a briefing in Kiev: "The likelihood of another possible invasion by Russian troops of Ukrainian territory is high ... it could happen at any moment."
In remarks published on Thursday, Ukraine's representative to the OSCE told an Austrian newspaper it was now hardly possible to speak of a ceasefire, citing 2,400 alleged breaches of the truce by rebels.
The ceasefire has been underpinned by a series of agreements known as the Minsk protocols, signed by Russia, Ukraine and rebels from the Ukrainian separatist regions of Donetsk and Luhansk.
"There is no intention of abandoning it (the ceasefire) despite all ... attempts to break these agreements by the Russian side," Ukrainian military spokesman Andriy Lysenko said.
(Additional reporting by Anton Zverev in Donetsk, Ukraine, Alessandra Prentice in Kiev, Writing by Thomas Grove and Alessandra Prentice; Editing by Janet Lawrence)