* U.N. investigators document latest violations
* Syrian forces blamed for most killings, torture
* Rebels also accused of executions, abductions (adds details, byline)
By Stephanie Nebehay
GENEVA, May 24 (Reuters) - Syrian government forces have executed entire families in their homes as part of a crackdown on the uprising against President Bashar al-Assad, U.N. investigators said on Thursday.
Both Assad's troops and opposition fighters were committing gross human rights violations despite a six-week-old ceasefire in the conflict, but the army and security forces were responsible for most of the crimes documented since March, a U.N. report said. Children were often victims, it said.
Government abuses included heavy shelling of residential areas, executions and torture. Syrian forces routinely drew up a list of wanted persons and their families before blockading and then attacking a village or neighbourhood, the report said.
Rebels, who are increasingly armed and well-organised, have executed or tortured captured soldiers and pro-government supporters, it said. They have also abducted civilians in an apparent bid to secure prison exchanges or ransoms.
"Most of the serious human rights violations documented by the commission in this update were committed by the Syrian army and security services as part of military or search operations conducted in locations known for hosting defectors and/or armed persons, or perceived as supportive of anti-government armed groups," the report said.
Children were frequently among those killed and wounded during attacks on protests and the bombardment of towns and villages by state forces, it said.
"Entire families were executed in their homes - usually the family members of those opposing the government such as the family members of Colonel Riad al-Asaad," it said, referring to the extended family of the head of the Free Syrian Army.
The team of investigators, led by Brazilian Paulo Pinheiro, has not been allowed into Syria but based its report on more than 200 interviews of victims and witnesses.
They confirmed 207 deaths during the two-month period. The United Nations has said that as of December, government forces had killed more than 9,000 people in the uprising against Assad that began in March 2011.
The world body is deploying up to 300 unarmed military observers in Syria to monitor an April 12 truce mediated by U.N.-Arab League envoy Kofi Annan that has yet to take hold.
Security forces used lethal force against demonstrations in Aleppo, Damascus, Deraa, Hama, Homs, Idlib and in numerous villages across the country since March, the report said.
"Other unlawful killings took place during government military operations undertaken to weed out defectors, anti-government armed groups, their families and other opponents perceived to be supporting anti-government armed groups."
Often Syrian forces issue a warning to hand over the wanted defectors or organisers of the anti-government protests, usually within a deadline, it said. Males in the area would hide or try to evacuate women, children and the elderly.
"Anyone seen trying to leave the area by avoiding the blockades were presumed to be members or supporters of anti-government armed groups and were shot," it said.
The U.N. panel said it had received multiple reports of the armed opposition executing members of the army and security forces, suspected informers and collaborators, and it gave details of two such incidents in Homs.
"A defector who fought in the ranks of Al Farouk Brigade ("FSA") in Homs city stated that members of the government forces, including what he claimed were three Iranian snipers, were summarily executed after they apparently confessed.
"One anti-government armed group fighter also admitted that he and his associates had killed government soldiers when the captives refused to join them," it said.
The U.N. panel has already drawn up a secret list of Syrian officials suspected of ordering crimes against humanity and given it to U.N. rights chief Navi Pillay. She has said that the situation in Syria should be referred to the International Criminal Court (ICC) for prosecution. (Reporting by Stephanie Nebehay, Editing by Angus MacSwan)