MOSCOW/ NOVOAZOVSK Ukraine (Reuters) - On Monday, a resident of Novoazovsk in south-eastern Ukraine said she saw a column of armored vehicles approach the town and start shooting.

"It all started at 8:00 this morning, tanks appeared, no fewer than seven of them," the woman, who gave her name only as Lyudmila, told Reuters by telephone. "Right now I can hear rumbling, explosions ... the residents are hiding."

In Kiev later that morning, Ukrainian officials said the column was an incursion by Russian troops which it alleges are fighting alongside pro-Moscow separatists, a claim Russia quickly dismissed as disinformation.

That is a now-familiar ritual: the five-month conflict over eastern Ukraine is one of claim and counter-claim by opposing sides, often centering on what role Russia is playing. With the battlefield mostly too dangerous for reporters to safely move around, verifying who is doing what is usually impossible.

On Tuesday, in a continuation of the pattern, Kiev said it had captured a group of Russian soldiers who had entered Ukraine on a "special mission", while Moscow said they were there by mistake.

However, the armored column that appeared on Monday in the far south-eastern corner of Ukraine, where it abuts the Russian border, was unusual because the spot was far removed from any territory held by the separatists.

It was therefore difficult to see how the column could have appeared in Ukraine without having come across the Russian border, unless it made an amphibious landing from the nearby Azov Sea which is improbable given the number of heavy vehicles witnesses said they saw.

A Reuters reporter was able to observe the situation in the area where the column was seen, first at the start of August and then most recently on Sunday afternoon, a few hours before the first sightings of the column were reported.

Those observations, combined with interviews with rebel leaders, Ukrainian soldiers, and other research, indicated two things.

First, that until late on Sunday there were no rebel formations within about 30 km (20 miles) of the area where the armored column first appeared, and had not been for weeks beforehand.

And secondly, that before the armored column appeared, the area had come under artillery fire at times when the nearest rebel positions were beyond the range of most types of weapon that could have delivered the strike.

It was not possible to establish whether the people driving the column and firing the artillery were Russian soldiers or separatist rebels. But there were strong indications that whoever it was doing those things operated out of Russian territory - something very unlikely to have happened without Moscow's consent.

The question of Russian involvement is at the core of Western governments' response to the Ukraine crisis, and could be crucial to how the conflict plays out.

The European Union and United States have already imposed sanctions on Russia in part based on allegations Moscow is arming the rebels. The West has warned of more sanctions if Russia provides further help.


ARTILLERY FIRE

A Ukrainian national guard unit stationed on the outskirts of Novoazovsk, on the road towards the Novoazovsk-Veselo-Voznesenka border crossing, showed a Reuters reporter a crater left by an exploding munition near their position.

They said the artillery fire was coming from over the border inside Russia, about 10 km (six miles) to the east.

"There were about 500 salvos from Grad (multiple rocket system) and mortars. There have been and are no rebels here whatsoever. They're just firing straight from Russia," Roman, the commander of a Ukrainian national guard unit, said on Sunday, before the armored column appeared in the same area.

Reuters saw no direct evidence of this, and Russian officials have repeatedly denied that their military is in any way involved in the conflict in eastern Ukraine, between pro-Moscow separatist rebels and government forces.