Syrian President Bashar al-Assad appears to be ignoring an ultimatum by his fellow Arab leaders, and what may have been their final attempt to toss out a life preserver before abandoning him to the fate of his late friend, Libyan strongman Muammar Qaddafi.

Sunday was the deadline for Syria to agree to a final invitation by the Arab League to sign -- and comply with - an agreement to end government violence against civilian protesters. But by late afternoon local time, Damascus had yet to reply to the Arab League's initiative, extended Saturday to the Syrian foreign ministry.

Meanwhile, at least 25 people died Saturday in clashes between government troops and a growing army of Syrian military defectors who have joined the movement to topple President al-Assad.

The protests, ignited by the region-wide "Arab Spring," began in March. But Assad's brutal attempt to repress the mostly peaceful demonstrations did nothing to stop them; if anything, it released tensions already simmering under the surface for far too long.

Members of the Arab League's ministerial committee gave Syria's government until Sunday to respond to the initiative, deciding in a meeting Saturday at Cairo headquarters to tighten sanctions if Damascus did not agree to end the violence.

Syria's membership in the Arab League was suspended in November due to Assad's refusal to fulfill his previous agreement to end the violence. The Syrian president, meanwhile, has been charged with crimes against humanity by the United Nations Human Rights Council, which ruled that he is personally responsible for the horrific actions of his troops against his people.

The ministers also confirmed the sanctions the regional body had approved in the past week, including the freeze on the assets of 19 high-ranking Syrian officials and Assad aides. The 19 were also added to a list of those included in a travel ban, and all transactions have been cut off with Syria's central bank.

In addition to the Arab League, the European Union, Turkey, the United States and Canada have imposed sanctions on Damascus, but to no effect.

Nearly all governments have urged their citizens to leave the country while there are still commercial flights available. All flights between Syria and its Arab neighbors are scheduled to cease by December 15.

The United Nations has estimated that some 4,000 people have died in the violence since the protests began in March. Activists and human rights organizations place the death toll higher, with figures climbing above 4,500 dead. November was the deadliest month so far, with close to 1,000 people killed in clashes, according to activist groups.