- NEW: Ultimatum lists a series of demands in a peace plan implemented last month
- Free Syrian Army does not say what will happen if the government fails to comply
- Syrian envoy says the government will finish its probe into a massacre this week
- China and Russia reiterate their rejection of any military intervention in the nation
(CNN) -- Syrian opposition fighters issued the government a Friday deadline to cease fire, pull out troops from residential areas and allow humanitarian aid.
The Free Syrian Army, which mainly comprises military defectors, did not say what will happen if the government fails to comply with the 48-hour deadline.
"Our national, moral and humanitarian duty make it necessary for us to defend and protect our civilians and their cities, towns, blood and dignity," the group said in a statement.
The ultimatum lists a series of demands in a peace plan implemented last month and brokered by United Nations special envoy Kofi Annan.
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U.N. monitors in Syria fired upon "Immediately halting gunfire and all violence, pulling out all the troops, tanks and machinery from residential areas, allowing humanitarian aid to reach all stricken areas, releasing all prisoners and allowing media access," said Col. Qasim Saad Eddine, the group's spokesman.
Eddine also called for freedom to demonstrate, an end to attacks on U.N. monitors in the nation and a dialogue on power handover.
Despite the peace plan, violence has continued almost daily, with international outrage growing over Friday's massacre in Houla that left more than 100 people dead, many of them children.
Susan Rice, the U.S. ambassador to the United Nations, said it was carried out by Shabiha militias or local gangs acting on behalf of the regime.
Syria has repeatedly denied government involvement. Bashar Jaafari, the nation's U.N. ambassador, said the government will finish its investigation into the massacre this week.
"And all of us will know for sure the identity of the perpetrators," he said.
Since the conflict began, the government has said "armed terrorist groups" are to blame for violence against civilians.
Alex Thompson, chief correspondent for Channel 4 News, who visited Houla expressed skepticism about the government's claim.
"The question you have to ask is, 'How was it 100 armed militia were able to come in and slaughter family after family, in an area which was an intensive shelling zone prior to them arriving, and yet when they came in the area, no shells fell on them?'" he told CNN's "AC360" on Wednesday night.
The new wave of killings highlight a conflict spiraling out of control as the call for President Bashar al-Assad's ouster that began in March 2011 has intensified into an armed insurgency.
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World reacts to Houla massacre The United Nations estimates 9,000 people have died since the protests began, while activist groups put the death toll at more than 11,000.
Clashes continue unabated, with regime forces and police shelling a Homs neighborhood Thursday, said the Local Coordination Committees of Syria.
A day earlier, at least 74 people were killed, said the Syrian Observatory for Human Rights, another opposition group
In addition, 13 bodies bound and shot were discovered in eastern Syria on Tuesday night, the head of the U.N. observer mission said. CNN cannot confirm death tolls or reports of violence from Syria because the government limits access to the country by foreign journalists.
In the wake of the latest violence, international pressure against the regime mounted.
Twelve nations -- the United States, the Netherlands, Australia, Britain, France, Germany, Italy, Japan, Spain, Bulgaria, Turkey and Canada -- have announced that they are expelling Syrian diplomats.
In retaliation, Syria told the Dutch charge d'affaires to leave.
Russia called the expulsions of Syrian diplomats "counterproductive," saying a U.N. Security Council statement this week condemning the incident was "a strong enough signal" to the warring sides.
China and Russia have reiterated their rejection of any military intervention in the nation. As permanent members of the U.N. Security Council, both have used their veto powers previously to block two resolutions against al-Assad, saying more balanced resolutions are needed.
CNN's Amir Ahmed and Faith Karimi contributed to this report.
