* Six UN staff spent night with rebels after gun attack
* Anti-Assad activists report new massacre in Homs
* Relentless violence challenges ceasefire
By Khaled Yacoub Oweis
AMMAN, May 16 (Reuters) - Six ceasefire monitors, who were caught overnight in the crossfire of Syria's civil conflict, were handed back to their U.N. colleagues on Wednesday by rebels fighting the forces of President Bashar al-Assad.
The stranding of the observers in a gun and bomb attack at a funeral that killed at least 21 civilians underscored the relentlessness of the violence challenging a U.N. ceasefire that is meant to lead to a peaceful resolution to the uprising.
"We gave the six with their cars to a U.N. convoy near the entrance of Khan Sheikhoun. They are all safe, in good heath and on their way to Damascus," Free Syrian Army commander Abu Hassan said by satellite phone from the site of the handover.
The observers stayed overnight with rebels who said they feared an assault by government forces after the funeral attack.
A pro-government TV station said unidentified gunmen opened fire at the funeral. The rebel commander said a pro-Assad militia was responsible and that his forces had the names of at least 27 people killed.
The head of the monitoring mission, Major-General Robert Mood, confirmed the monitors were heading back to base.
"They have departed from Khan Sheikhoun and are on their way back. They expressed to me that they have been well treated," he told reporters in Damascus.
He expressed gratitude to the Syrian government for "facilitating coordination" for the exit of the observers, and to the people of Khan Sheikhoun, about 220 km (140 miles) north of Damascus, for treating them "with respect."
"That kind of violence is obviously the kind of violence we don't want to see," he said. "It is not going to contribute constructively to the aspirations of the Syrian people."
ACCOUNT OF NEW MASSACRE
The handover came as a Britain-based opposition group, the Syrian Observatory for Human Rights, said at least 15 people had been killed since Tuesday when security forces stormed the Shammas district of Homs, parts of which Assad's forces reduced to rubble with artillery fire earlier this year.
The group said security forces carried out summary executions in the city. Footage distributed on YouTube showed bodies - some with what looked like gunshot wounds - purported to be those killed during raids in the city.
There was no independent confirmation of the claims from within Syria, which has restricted journalist access during the 14-month-old uprising.
The Free Syrian Army has a nominal leader based in Turkey and tenuous ties with the divided political opposition group, the Syrian National Council (SNC), which on Tuesday re-elected Burhan Ghalioun, a sociologist long resident in France, as its leader for another three months.
People involved in the vote, which took place in Rome, said the secular Ghalioun was viewed as acceptable to Syria's array of sects and ethnic groups, and to major factions of the umbrella SNC which seeks recognition as the sole legitimate representative of opposition to Assad.
Shortly afterwards, Fawaz Tello, a prominent dissident, resigned from the SNC, the latest of several senior figures to quit the body in recent months.
As the SNC debated its leadership, Damascus announced the results of parliamentary elections it points to as proof of Assad's determination to resolve the uprising peacefully.
(Additional reporting by Mariam Karouny; Writing by Joseph Logan; Editing by Robin Pomeroy)
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