Just when it looked like Syrian President Bashar al-Assad was taking a step towards cooperation, new reports have surfaced of mass killings on the eve of a visit from Arab League observers.

Today the Syrian National Council, an opposition alliance seeking to topple President Assad, emailed a statement claiming state forces have killed 250 people in the past 48 hours in the regions of Idlib, Homs and al-Zawiyah, and called for a United Nations’ Security Council meeting to address alleged “massacres” and “large-scale genocide.”

“I fear the security forces may be trying to crush this thing before the monitors get in," said Murhaf Jouejati, a member of the Syrian National Council, an opposition group.

The Syrian government has not commented on the reports, but the official Syrian Arab News Agency said on its web site that Syrian authorities in the cities of Idlib, Homs and Daraa had “stormed dens of armed terrorist groups, arresting tens of wanted men who committed crimes of killing, attacked and sabotaged private and public properties.”

Observers from the Arab League are to visit Syria tomorrow for the first time to monitor pledges by Assad’s government to withdraw its troops from besieged areas. About 500 observers are set to deploy around Syria in groups of 10, Arab League Secretary-General Nabil el-Arabi said yesterday in Cairo.

“The words of the Assad regime have no credibility when they continue to be followed by outrageous and deplorable actions,” wrote White House Press Secretary Jay Carney in a released statement. “Only two days following the Assad regime’s decision to sign the Arab League initiative, they have already flagrantly violated their commitment to end violence and withdraw security forces from residential areas.”

Carney's statement notes that the U.S. “continues to believe that the only way to bring about the change that the Syrian people deserve is for Bashar al-Assad to leave power.

According to a United Nations estimate, more than 5,000 people have been killed in Syria since March.