By Maria Tsvetkova

DONETSK Ukraine (Reuters) - A 37-year-old rebel fighter in eastern Ukraine is speaking out against the separatist leadership, accusing them of adopting flashy lifestyles and pushing locals aside in a rebellion that has largely been taken over by Russians.

The online post by Aleksander reflects simmering discontent among local people backing the self-proclaimed "Donetsk People's Republic" over the sidelining of its Ukrainian-born leaders. Interviewed by Reuters, Aleksander declined to give his full name for fear of reprisal.

While the emerging strains in the rebel ranks do not reflect a change of heart in their bid to carve out autonomous pro-Russian enclaves in Ukraine, they do threaten the separatists' cohesion at a time when Kiev's troops are taking back ground.

"What are we fighting for?," asks Aleksander in the July 31 post, which tells of frustration and anger in the lower rebel ranks and warns separatist leaders not to betray the cause lest one day fighters turn their weapons on "the enemy within".

"Many members of the new authorities are disdainful and arrogant towards the rank and file, cruise in expensive cars, wear expensive suits, use expensive weapons and phones, demonstrating... their material and moral superiority," it said.

"Meanwhile, we, regular volunteer militants, rose to defend our native land with the hope of building a new country, a more just society."

Wearing a camouflage uniform and touting a Kalashnikov rifle, Aleksander told Reuters there was in fact "no political leadership" left in the separatist movement after locals, including the "people's governor" of Donetsk Pavel Gubarev, were pushed aside by Russians.

He was speaking in Donetsk, the largest city in the hands of the rebels where they have vowed to make a stand against government forces. Security officials in Kiev say their troops are preparing to "storm" the city.

Tensions among the rebels have been growing since government troops pushed them out of Slaviansk, a former bastion of resistance, on July 4-5.

Locals including Gubarev have been increasingly sidelined from decision-making or dismissed since then and more Russians have joined the fray - a response to slow but steady progress made by the Ukrainian army and volunteer battalions on the ground.

Three top leaders of the rebellion now - Aleksander Borodai, Igor Girkin also known as Strelkov, and Vladimir Antyufeyev - are Russian. Kiev says another prominent and feared field commander, Igor Bezler, is a Russian intelligence operative.

STRAINS

Kiev's military operation against the rebels has gained momentum since President Petro Poroshenko was inaugurated in early June.

Fighting intensified further after a Malaysian airliner was downed over rebel-held territory on July 17, killing all 298 on board, with Kiev and Western countries blaming the separatists.

The rebels have now been pushed back towards their main strongholds of Donetsk and Luhansk, which they together call Novorossiya (New Russia).

Antyufeyev - a senior separatist figure and the latest Russian-national to join the Donetsk People's Republic (DNR) leadership - said he was not aware of the post. Its author might have been paid by Kiev to write it and he ought to go off and fight instead, he said.

A heated debate that developed on the website after the post appeared suggests separatist leaders should not dismiss it too easily, however, though it is not clear how popular it is among other rebels.

Aleksander, who has a patch proclaiming allegiance to Novorossiya stitched to his uniform, said a unit of several dozen men in which he serves and which guards one of the rebels' headquarters in Donetsk had mostly backed his stance.

He was careful not to single out any specific member of the leadership for criticism.

But his post read: "We do not want, at the expense of our blood and lives, under the cover of patriotic slogans, to see a simple switch in property ownership from the Ukrainian business elites to a new elite that is already forming".

The statement highlights how poor prospects and frustration among natives of this impoverished industrial region helped fuel the pro-Russian separatist revolt.

Aleksander, who is from the port city of Mariupol that Kiev troops reclaimed from rebels in June, told Reuters with a grim smile he may end up being sorry for crossing the line of tight discipline imposed by some rebel commanders.

(Writing by Gabriela Baczynska, editing by Richard Balmforth and Janet McBride)