By Stephanie Nebehay and Mariam Karouny
GENEVA/BEIRUT, May 24 (Reuters) - A United Nations investigation said Syrian government forces and rebels trying to topple President Bashar al-Assad had both committed serious human rights abuses despite an attempted ceasefire in the conflict and opposition activists reported fighting in several regions on Thursday.
The investigators' report said government forces had killed entire families, and their insurgent foes had kidnapped for ransom and tortured prisoners.
Separately, a Syrian Islamist said he was trying to broker the release of Lebanese Shi'ites kidnapped by Sunni insurgents fighting Assad's forces, the latest strand of Syria's conflict to entangle its smaller neighbour.
In Damascus, Assad told a visiting Iranian minister that Syria would recover from the unrest.
"Syria was able to overcome pressure and threats which it was exposed to for years, and with the steadfastness of its people and their adherence to unity and independence it will be able to come through the present crisis," he told Iranian Telecommunications Minister Reza Taqipour
Violence has raged despite a U.N.-brokered agreement on April 12 aimed at halting the bloodshed in Syria, where Assad is confronting a 14-month uprising which began with peaceful protests but has become increasingly militarised.
The report by U.N. investigators - who were not allowed into Syria and relied on interviews of victims and witnesses - said government forces routinely drew up lists of wanted people and their families before blockading then attacking a village or neighbourhood.
"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 relatives of the head of the rebel Free Syrian Army.
The rebels, who are increasingly armed and well-organised, have executed or tortured captured soldiers and government supporters, it said. They have also abducted civilians in an apparent bid to secure prison exchanges or ransoms.
The report, which documented 207 deaths since March, said children were frequent casualties of attacks on protests and the bombardment of towns and villages by state forces. Investigators also had multiple reports of insurgents executing suspected collaborators and captured government troops.
The report was released as an opposition group, the British-based Syrian Observatory for Human Rights, said Syrian forces executed four people taken from their homes and killed four others during clashes in northern Idlib province.
There was no independent confirmation of the account from Syria, which has limited journalist access in the uprising.
The United Nations, Syria's close ally Russia and Saudi Arabia, which has called for Assad's departure, have all expressed concern the Syrian conflict will draw in Lebanon and tip its delicate sect-based politics into civil war.
The latest hint of spillover came with the abduction this week of 13 Shi'ite Muslim pilgrims in northern Syria. A Syrian Islamist cleric told Reuters he was negotiating their release.
"They are well and safe, we are trying to secure their release, but the Syrian army shelling of the area has been blocking it so far," Sheikh Ibrahim al-Zoaby, head of the Free People of Syria group, told Reuters.
Zoaby said the kidnappers want to hand the men to the Lebanese authorities.
The head of Hezbollah, the Shi'ite Muslim guerrilla and political movement that is Syria's most powerful Lebanese ally, has appealed for calm over the kidnappings, which provoked protests in largely Shi'ite districts of Beirut, which has seen its worst unrest since 2008 this week.
Sunni Muslim factions opposed and loyal to Assad fought street battles with machineguns and rocket-propelled grenades that left at least two people dead, after the killing of an anti-Assad cleric in northern Lebanon by Lebanese troops ignited violent protests that spread to Beirut.
North Lebanon, a stronghold of conservative Sunnis who back the revolt against Assad, saw his Lebanese Islamist foes clash with Lebanese troops earlier this month after the arrest of an anti-Assad Islamist.
That fighting later expanded to include members of the minority Alawite sect to which Syria's rulers belong, and killed nine people before Lebanese judicial authorities freed the detained man on bail.
Assad has ruled for 12 years after succeeding his late father Hafez al-Assad. Once seen as potential reformer, he has clung to power in the face of the revolt despite international condemnation of the goverment crackdown on the opposition and calls for him to step down.
His soldiers and security forces have killed more than 9,000 people in a campaign to quell the uprising, according to the United Nations. The government says rebels have killed more than 2,600 military and security personnel. (Writing by Dominic Evans; Editing by Angus MacSwan)