WASHINGTON -- The resignation Thursday of veteran diplomat Kofi Annan and the collapse of diplomatic efforts on Syria by the United Nations and the Arab League all but assure a bloody finish to the uprising against President Bashar Assad.

Annan’s tenure as mediator yielded several abortive attempts to wring a peaceful transition from what’s become a civil war with dire regional implications, including an expanding refugee crisis and a new battleground for militant Islamists.

The envoy’s pleas for both sides to stop fighting and start negotiating were seen as so ineffective that Syrian activists on Twitter joked Thursday, “Annan resigned? From what?” Both U.N. Secretary-General Ban Ki-moon and Susan Rice, the U.S. ambassador to the U.N., issued statements that used the word “thankless” to describe Annan’s unenviable position between the warring parties.

Annan told a news conference in Geneva that some had dubbed his task “Mission Impossible.” He blamed all sides for the breakdown in diplomacy, which he said rendered his role ineffective.

“The bloodshed continues, most of all because of the Syrian government’s intransigence and continuing refusal to implement the six-point plan,” Annan said, “and also because of the escalating military campaign of the opposition – all of which is compounded by the disunity of the international community.”

Through 17 months of political isolation, harsh sanctions and insurgent attacks, Assad has clung to power. Annan’s departure, coupled with the refusal so far of the United States and other powers to intervene militarily, turns the fight to unseat him into a war of attrition between the ragtag rebels and the regime’s bloodied-but-resilient forces.

Thursday brought continued fighting across the country, including in the capital, Damascus, and in Aleppo, the country’s largest city and its economic hub. The Syrian government reportedly was sending reinforcements to Aleppo as it attempted to dislodge rebels there from a number of neighborhoods that had been used to launch attacks on security forces across the city.

“The conflict continues to spiral out of control, with shocking atrocities and human rights abuses coming to light almost every day and now being committed by both sides,” Suzanne Nossel, the U.S. director of the human rights group Amnesty International, said in a statement.

Nossel called Annan’s resignation the “culmination of a string of failures” by the international community to rein in the violence.

Secretary-General Ban announced Annan’s departure in a brief statement that praised him for “the selfless way in which he has put his formidable skills and prestige to this most difficult and potentially thankless of assignments.” The statement added that Ban and Arab League chief Nabil al Araby were in talks to pick a successor.

Many Syrian opposition activists criticized Annan’s plan from Day One, calling it a ploy to buy more time for both Assad’s regime and the U.S. administration, which is loath to take risky action in Syria with a presidential election looming.

Most Syrians who are opposed to Assad’s government never held much faith in Annan’s efforts to reach a peaceful agreement with a government in which they’d long ago lost trust. Others feared that Syria finally had fully become a proxy battleground for the interests of outside countries.