Human Rights Watch says Syrian government forces have summarily executed more than 100 people, most of them civilians.
In a report released on April 9, the New York-based group said this includes several mass executions in the restive provinces of Homs and Idlib.
The group claims it only included cases corroborated by witnesses but has received more reports of similar incidents.
The executions took place over the past four months, with most occurring in March.
An internationally-brokered plan to stop 13 months of bloodshed in Syria appeared set to fail after President Bashar al-Assad's government raised new, last minute demands that the country's largest rebel group swiftly rejected.
The ceasefire plan brokered by UN-Arab League envoy Kofi Annan was supposed to go into effect on April 10.
France Says New Demands 'Unacceptable'
France meanwhiled has condemned Damascus's new demands, which it calls "unacceptable."
The statement from the French Foreign Ministry comes after Damascus said it wants the rebels to first give written guarantees that they will stop fighting before government troops pull back.
Damascus also wants foreign governments to stop funding the opposition.
Syria said that UN-Arab League envoy Kofi Annan had made a "wrong interpretation" of its acceptance of the cease-fire.
Speaking to Reuters, Free Syrian Army (FSA) chief Riad al-Asaad accused Syrian President Bashar al-Assad of reneging on the deal and predicted that the agreement is doomed to fail.
But Colonel Qassem Saad al-Deen, spokesman for the joint command of the FSA inside Syria, said the rebels would respect the cease fire deadline.
Escalating violence has already raised questions over the cease-fire which was formally endorsed by the UN Security Council on April 5.
A day later, however, Damascus said the number of "terrorist acts" had risen since the deal with Annan was agreed.
Opposition activists said dozens of people were killed and wounded on April 8 when Syrian forces shelled a rebellious area near the border with Turkey.
Activists said rebel fighters shot down a Syrian army helicopter with a heavy machine gun in the northwestern Idlib province.
Pope Benedict used his Easter Mass to call for an end to the violence in Syria, which the UN says has claimed some 9,000 lives since it erupted more than a year ago.
Annan To Visit Refugees, Iran
Annan, meanwhile, will visit a camp for Syrian refugees in Turkey on April 10.
Annan's spokesman Ahmad Fawzi told Reuters news agency in Geneva that Annan will visit a refugee camp in Hatay province on his way to Iran where he is due to hold talks on April 11 with senior officials in the Tehran administration, which is a key ally of Syria.
Turkey says some 24,000 Syrian refugees are sheltering in camps it has set up in the southern provinces of Hatay and Gaziantep.
Turkish officials said two Syrian refugees and one Turkish translator were wounded on April 8 when the Kilis border refugee camp in Gaziantep came under fire from the Syrian side of the border.
The Turkish authorities also said two of 13 Syrians who were wounded in clashes inside Syria and were brought to Kilis for treatment earlier on April 8 have died
With reporting by AP and Reuters