* Group of 28 officers facing trial for May coup bid
* Experts fear patchwork army could splinter
* Government says troops united, no ethnic rifts
* Burundi accuses Rwanda of allowing rebels to recruit (Adds U.S.-backed call to send investigators to Burundi)
NAIROBI, Dec 14 (Reuters) - More than two dozen generals and senior army officers accused of being behind a failed coup went on trial in Burundi on Monday amid heightened tensions in the capital after attacks last week by insurgents in which about 90 people were killed.
Former defence minister Cyrille Ndayirukiye and five other generals are among the 28 people standing trial for their role in the attempted coup in May, launched when President Pierre Nkurunziza was abroad. The coup was swiftly foiled.
Onésime Kabayabaya, one of the defence lawyers, told Reuters the trial had begun in the central town of Gitega. He said the defendants had complained about mistreatment in jail and said they had not had time to review their case files.
Justice Ministry spokeswoman Agnès Bangiricenge said the group was "charged with an attempt to unseat the country's constitutional institutions", as well as carrying out assassinations and other acts of violence.
The trial highlights what experts say are worrying signs of division among the security forces in a crisis that erupted in April when Nkurunziza announced his bid for a third term.
Opponents said his re-election, secured in a disputed vote in July, violated a peace deal that ended a civil war in 2005.
That peace deal, which opponents said limited the president to two five-year terms, also included reforms to the army, which absorbed rebel fighters of the majority Hutu ethnic group into a force that had been led by the Tutsi minority.
Burundi's government insists the army remains united, but experts fear the violence could fracture the patchwork force.
The crisis has stoked tensions in a region where memories of Rwanda's 1994 genocide are still raw. Burundi accused Rwanda, which has the same ethnic mix, of letting rebels recruit Burundian refugees on its soil, a charge Kigali denied.
The U.N. Human Rights Council will consider on Thursday a U.S.-led resolution to send United Nations investigators to Burundi to probe abuses, including the use of live ammunition against protesters and "targeted assassinations", the U.S. text said.
The trial follows fighting in the capital Bujumbura last Friday when insurgents attacked military sites. The government said the gunmen had aimed to seize weapons but had failed.
"Both Hutu and Tutsi in the army were united to kick off the attackers," presidential media adviser Willy Nyamitwe told Reuters, dismissing reports that soldiers in the camps had fought each.
He said the attackers "came from outside the barracks".
Following a typical pattern during the months of crisis, life in Bujumbura swiftly returned to a semblance of normality after Friday's flare-up, with people back on the streets and shops reopening.
The violence was just the latest in a spate of shootings, explosions and assassinations.
Several leaders in the May coup bid fled. One of them said the plotters still planned to overthrow the president. Burundi has accused Rwanda of providing safe haven for the rebels.
Burundi's Nyamitwe said Kigali was allowing Burundian refugees to be recruited "into a rebel group, trained and armed by Rwanda", making his comments after a charity said in a report that "non-state armed groups" were recruiting.
Rwanda, which hosts more than 73,000 Burundian refugees, denied the allegations.
More than 220,000 Burundians have fled their country, heading to neighbouring states, as the violence persists. (Additional reporting by Ed Cropley in Johannesburg, Clement Uwiringiyimana in Kigali and Stephanie Nebehay in Geneva; Writing by Edmund Blair; Editing by Gareth Jones)
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