* Video is further blow to rebels' image
* U.N. says it could be used as evidence of war crime
* Alleged atrocity is not first by rebels
GENEVA, Nov 2 (Reuters) - A video that appears to show Syrian rebels killing soldiers who had surrendered probably constitutes a war crime that should be prosecuted, the United Nations human rights office said on Friday.
The disturbing incident looks to be the latest atrocity committed by opposition fighters seeking to topple President Bashar al-Assad, Rupert Colville, a U.N. rights spokesman, said.
"It will be examined carefully," Colville told a news briefing in Geneva, saying it may be difficult to identify the perpetrators and the location where the footage was recorded.
"But the allegations are that these were soldiers who were no longer combatants. And therefore, at this point it looks very likely that this is a war crime, another one," he said.
The footage, which has been widely publicised around the world, has dealt a further blow to the rebels' image and is embarrassing for their foreign supporters.
Anti-government rebels killed 28 soldiers on Thursday in attacks on three army checkpoints around Saraqeb, a town on Syria's main north-south highway, a monitoring group said.
Some of the dead were shot after they had surrendered, according to video footage. Rebels berated them, calling them "Assad's Dogs", before firing round after round into their bodies as they lay on the ground.
"Unfortunately, this could be just the latest in a string of documented summary executions by opposition factions as well as by government forces and groups affiliated with them, such as the shabbiha (pro-government militia)," Colville said.
He told Reuters: "A year ago, reports of excesses and atrocities by opposition forces were few and far between. But towards the end of last year and increasingly during 2012, we've seen events like this happen."
More than 32,000 people are estimated to have been killed since protests against Assad first broke out and then degenerated into full-scale civil war, with his forces using artillery and air strikes.
U.N. human rights investigators led by Brazilian Paulo Pinheiro have gathered evidence and testimony on atrocities committed by government forces as well as by armed rebels in the 19-month-old conflict.
In their last report in August, the investigators said government forces and allied militia had committed war crimes including the murder and torture of civilians in what appeared to be a state-directed policy.
The team has already drawn up a confidential list of suspects for future prosecution, either by an international tribunal or by a Syrian national court if feasible.
Carla del Ponte, the former U.N. war crimes prosecutor who has joined the inquiry, promised last month to bring high level Syrian political or military figures who may have ordered or committed war crimes to justice.
"There should be really no illusions that accountability will follow. There is a lot of evidence for many of these crimes that have been taking place," Colville said on Friday.
"This video, if it's verified, if the details become a little clearer, could well be part of that evidence," he said.
Colville called once again on both sides to respect international human rights law and international humanitarian law. "You can carry out conflict within the rules of war".
The Geneva Conventions lay down rules that aim to limit the barbarity of war. They call for the protection of civilians and of those who no longer take part in hostilities, including captured soldiers and the wounded, who it says should be humanely treated in detention.
The International Committee of the Red Cross (ICRC), which is the guardian of the 1949 humanitarian pacts, declined comment on the video on Friday, saying it had no firsthand information.