GENEVA â€"  Syrian government forces and anti-government armed groups are both carrying out unlawful killings, torturing opponents and abusing children in the 15-month-old uprising, a U.N.-appointed panel of human rights experts said Thursday, though it underlined that security forces are still responsible for the largest share of the violence.

The findings by the Independent International Commission of Inquiry on Syria show a chilling pattern of abuses on both sides of a conflict that it says has become "increasingly militarized" despite U.N. cease-fire efforts. The report is based on hundreds of interviews since March with victims and witnesses who fled the country.

The three-member panel says the conflict has shifted and the government now faces armed and well-organized fighters bolstered by defectors. The widespread human rights abuses by government forces occur "most often during large-scale, military attacks on specific locations known for hosting defectors and other anti-government sympathizers," it says.

The report documents unlawful killings by government forces in Idlib, Homs, Aleppo, Hama, Damascus and Daraa and in numerous villages. It says a clear pattern has emerged in which an attack is preceded by a blockade of main roads in a neighborhood or village.

The panel also cites accounts of security forces using both precise shelling to hit opposition stronghold and indiscriminate targeting of residential areas where opposition fighters are believed to be hiding.

After the shelling, it says, security forces go in, backed by snipers on rooftops often using school buildings, and search house-to-house. The reports says that in some cases, suspects and others were executed immediately, according to repeated accounts.

"Fighters in anti-government armed groups were killed after being captured or wounded. In some particularly grave instances, entire families were executed in their homes -- usually the family members of those opposing the government," it says.

The reports says government forces have resorted to use of beatings and electric shocks on captured opponents and killed children by sniper fire. Children, including boys as young as 10, who have been detained by government forces, it says, have repeatedly said they are "tortured to admit that older male members of their fammly are Free Syrian Army
soldiers or supporters."

In March, a school in Atarib was occupied by government tanks, with snipers posted on its rooftop, and another school in a nearby village was burned down because of rumors its headmaster was an anti-government sympathizer, the report says. In April, the report says, a village school in Hama was used as a government command post with snipers on the roof.

The commission is chaired by Brazilian professor Paulo Sergio Pinheiro and includes Karen AbuZayd, who formerly headed the U.N. agency for Palestinian refugees, and Yakin Erturk, a former U.N. investigator on violence against women.

It has published previous reports on the conflict that the U.N. says has killed well over 9,000 people. In March, the commission handed U.N. human rights chief Navi Pillay a secret, sealed list of top Syrian officials who could face investigation for crimes against humanity carried out by Syria's security forces against government opponents.

But, the report says there is a growing list of abuses committed by anti-government armed groups, including executions of military forces and suspected informers. It says the opposition has increasingly resorted to improvised explosive devices.

The panel says it has videotaped sessions in which captured Syrian government forces or supporters are seen confessing under torture, with signs of bruising and bleeding. Anti-government armed groups also have abducted civilians and government forces apparently for prisoner exchanges or ransom.

Children are also being used as messengers and cooks, or to deliver medical supplies to field hospitals. The panel says its staff met children who regularly traversed the Turkish-Syrian border. Four had earlier been injured by sniper fire.

Syria's main opposition council, meanwhile, said it has accepted the resignation of its Paris-based president who earlier offered to step down amid mounting criticism of his leadership.

The executive committee of the Syrian National Council asked Burhan Ghalioun to pursue his duties until a new president is elected at a meeting on June 9 and 10.

The SNC has been plagued by infighting and divisions since its inception in September, complicating Western efforts to bolster the opposition. Ghalioun's offer to resign came just days after he was re-elected for a third, three month term in a controversial vote in Rome.