Syria's major opposition group condemned Bashar al-Assad's regime Wednesday for "brutal massacres" this week and urged the UN Security Council to protect civilians against "acts of genocide."

"The regime is using children as human shields so that tanks and armored vehicles can storm residential areas," said the Syrian National Council (SNC), which said that about 250 people have died over a 48-hour period.

"Incidents of gruesome murders have been recorded," the council said Wednesday, December 21, including the killings of four brothers and the beheading of a sheikh, whose head was hung above a mosque entrance. Both incidents occurred in Idlib, and the council says "acts of genocide" are occurring in Zawiyah Mountain in Idlib province in the northwest and the city of Homs in the west.

It cites "the regime's use of heavy weapons and artillery in shelling civilian neighborhoods, as was the case in the Baba Amr neighborhood of Homs, as well as in the villages of Kansafra, Kafar Awaid, and Mazrain, where a large number of residents were killed. Other residents were forced to flee under heavy gunfire," the council said. The villages are in Idlib province.

The violence spiked as Syria agreed to an Arab League observer mission Monday aimed at ending the violence between regime forces and protesters that started in mid-March. The United Nations this month estimated that about 5,000 people have died in the bloodshed.