DONETSK Ukraine (Reuters) - International monitors deployed along the Russian border in eastern Ukraine say their drones were shot at and jammed days before new columns of unmarked soldiers and weapons, said by the West to be Russian, were seen in the rebel-held territory this week.

The reports by the Organization for Security and Cooperation in Europe (OSCE), which is seen as a neutral party in the conflict that has killed more than 4,000 people, come as Kiev and Moscow trade accusations of violating a fragile ceasefire.

But Western officials say the rebels are behind the incidents, and Kiev says the rebels have intentionally hampered OSCE monitoring efforts to cover up the arrival of new Russian reinforcements of soldiers and weapons in eastern Ukraine, boosting fears of a return to hostilities.

“The fact that the separatists are jamming the (drones) or shooting at them to some people is evidence that they have something to hide,” said U.S. Ambassador to the OSCE Daniel Baer.

Russia denies sending weapons and soldiers to eastern Ukraine and says the reports are part of an information war against the Kremlin.

In its reports, the OSCE did not say who carried out the jamming. Ukraine denied involvement while a rebel commander, Eduard Basurin, said it was unclear who had shot at the drones.

Pro-Russian rebels have controlled large swathes of the border between Russia and Ukraine since the start of the conflict, which has free movement across the line.

On Nov. 3, the OSCE said one of its drones was jammed by military grade equipment near the town of Chermalyk in southeastern Ukraine, 30 km (20 miles) from the border.

A day earlier, another drone was shot with an anti-aircraft gun by a man in camouflage while it was flying over a separatist checkpoint 20 km (12 miles) east of the port of Mariupol.

“The militants are simply shooting at the drones, which are trying to monitor the situation on the border and areas of fighting. The militants are not interested in the world knowing the truth,” said Vladislav Seleznyov, spokesman for Ukraine’s military operations in the east.

“Putin says there are no Russian troops in Ukraine, but it’s not true,” he told Reuters by telephone.

Reuters saw a 50-vehicle column armed with rocket launchers and artillery guns traveling toward the rebel stronghold of Donetsk in eastern Ukraine from the direction of the Russian border on Tuesday.

The OSCE has also reported on several other columns of armored vehicles, guns and soldiers without insignias moving westward from the direction of the border.

Rebel commander Basurin in Donetsk, eastern Ukraine, the political and military hub for the rebels, said 2,000 Russians were in the rebel ranks but they were all volunteers who had come of their own free will.

“There are many people who are in uniform who finished service, who are officers, soldiers, contract soldiers ... But it doesn’t mean they are Russian soldiers,” said Basurin inside the complex he uses as a base.

Outside the gates of the complex, stood several men armed with automatic rifles, speaking in Russian without a Ukrainian accent. One had a patch with the Russian flag on his arm.

Russia has grown increasingly critical of the OSCE, saying that its reports have grown one-sided.

“We call on the leadership of the special mission to abandon a role of being ‘a witness of the prosecution’ for Kiev and get back to work on an impartial basis,” Foreign Ministry spokesman Alexander Lukashevich told journalists.