The United Nations says more than 3,500 people have been killed since mid-March in the regime's crackdown on dissent.
"It is crucially important now that President Assad immediately stop killing his own people."
The United States meanwhile urged Arab leaders to step up pressure on Syria ahead of Wednesday's meeting at which the suspension of Damascus takes effect.
"We look for the Arab League tomorrow to again send a forceful message to Assad that he needs to allow for a democratic transition to take place and to end the violence against his people," State Department spokesman Mark Toner said.
Syria, in an apparent last-minute show of goodwill, released 1,180 prisoners who were arrested during the anti-regime protests, in line with one of the points of the Arab League plan.
"1,180 prisoners who had been involved in the incidents in Syria and who did not have blood on their hands were released today," Syrian state television reported.
Top dissident, Kamal Labwani, who was jailed for 12 years in May 2007, was also reportedly freed.
The foreign ministry, in a statement on state television, said Syria will not attend the talks in Rabat, during which Turkish leaders would also be present.
"Syria's decision to attend the Rabat meetings as due to the wish of some Arab countries but in light of the statements made by Moroccan officials Syria decided it will not attend," said the statement without further details.
Turkey meanwhile took its growing impatience with Syria to a new level slamming Damascus with unprecedented threats of economic sanctions.
And Turkish Prime Minister Recep Tayyip Erdogan, once a close political ally and a personal friend of Assad, said he had lost hope that the Syrian leader would back down from saving his regime through bloodshed.
"A future cannot be built on the blood of the innocent, otherwise history will remember those leaders as ones who feed on blood," said Erdogan. "And you Assad, you are now coming closer to opening that page of history."
Ankara hit Syria with energy sanctions, halting joint oil explorations and threatening to cut power supplies to its southern neighbour which has been struggling with electricity shortfalls for the past two years.
Energy Minister Taner Yildiz also said Turkey's Petroleum Corporation had stopped exploration with Syria's national oil company in six wells.
The United States welcomed Ankara's moves.
"We very much welcome the strong stand that Turkey has taken. It sends a critical message to President Assad that again he cannot crack down and repress the aspirations of his people," deputy national security adviser Ben Rhodes said.
On November 2, Syria agreed to an Arab League roadmap to end the violence within two weeks but it failed to keep its end of the bargain, prompting the 22-member bloc to vote to suspend its membership.
Turkey's moves against Syria came in the wake of weekend attacks on Turkish diplomatic missions by pro-regime protesters angry over Ankara's support for the Arab League suspension vote.
The embassy attacks fuelled the anger of the 15-member UN Security Council who issued a statement Tuesday condemning the attacks "in the strongest terms" and calling on Syrian authorities to respect their international obligations.
Jordan's embassy was also stormed by regime supporters overnight after King Abdullah II became the first Arab leader to publicly call for Assad to quit, exacerbating Syria's growing isolation.
"I believe, if I were in his shoes, I would step down," the king told the BBC on Monday.
Monday was one of the bloodiest days of Syria's uprising that saw more than 70 people killed, rights groups said.
There was more bloodshed on Tuesday with two civilians dead, including a child, the Syrian Observatory for Human Rights said, adding five soldiers were killed in clashes with army deserters.
The fighting erupted in the town of Hara in Daraa province, where the unprecedented protests, which initially urged democracy before turning roundly against Assad's 11-year reign, first broke out, it said.
In northwestern Idlib province bordering Turkey "clashes between the regular army and armed men, probably deserters, caused at least 14 casualties -- dead and wounded," the Britain-based watchdog added.
Regime opponents who are demanding that Assad quit attempted on Tuesday to persuade Russia to toughen its stance against the regime in talks in Moscow with Russian Foreign Minister Sergei Lavrov.
But Lavrov was not swayed, said Burhan Ghalioun who heads the Syrian National Council, the largest and most representative opposition group to have emerged since mid-March.
Ghalioun said "it is necessary for Russia and the international community to send an important signal and demand Bashar al-Assad's resignation."
Russia, one of Syria's main arms suppliers, last month joined China in vetoing a UN Security Council resolution spearheaded by the United States and the European Union that would have threatened "targeted measures."