Activists in Syria say security forces shot dead 111 civilians on Tuesday in the north-western province of Idlib, in a second day of heavy fighting, Agence France Presse reported.
More on GlobalPost: Dozens of Syrian army defectors 'shot while fleeing'
The civilian deaths come as 100 army deserters were reportedly killed by machine gun fire as they attempted to flee their base in the same province.
Other fighting took place in the central regions of Homs and Hama and in the southern province of Deraa, the BBC reported.
The White House condemned the attacks on Wednesday in a statement, blaming Syrian President Bashar al-Assad for "flagrantly" violating a recent Arab League initiative the regime agreed on to end the nine-month-long violence.
"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," the statement said. "The United States is deeply disturbed by credible reports that the Assad regime continues to indiscriminately kill scores of civilians and army defectors, while destroying homes and shops and arresting protesters without due process."
The violence comes days before a team of Arab League observers is due to arrive in the capital, Damascus.
The league said Samir Seif al-Yazal, its Sudanese Assistant Secretary General would head an advance team, which will oversee the implementation of a peace deal.
Arab League chief Nabil el-Araby told Reuters the rest of the team of some 500 monitors would arrive by the end of December.
More on GlobalPost: Syria agrees to let Arab League observers oversee peace deal
After weeks of stalling, Syria signed the deal to accept monitors on Monday after the Arab League threatened to ask the UN Security Council to adopt its peace plan for Syria, which increased the chances of international action, Al Jazeera reported.
Meanwhile the Gulf Cooperation Council on Tuesday urged Syria to immediately end what it called its "killing machine."