WASHINGTON (Reuters) - U.S. Secretary of State John Kerry accused Syrian President Bashar al-Assad’s government of carrying out “nothing short of a massacre” in Aleppo, where thousands on Thursday were evacuated under a ceasefire deal from the last rebel enclave in a city besieged by fighting for years.

United States Secretary of State John Kerry delivers a statement on the situation in Aleppo, Syria at the State Department in Washington U.S., December 15, 2016. REUTERS/Gary Cameron

Kerry vigorously defended U.S. diplomatic efforts to end the war - all of which have been futile - and in which Assad, backed by Russia, Iran and Shi’ite militias, has gained the upper hand in the latest turn of the nearly six-year-old conflict.

Ultimately, Washington was forced to watch from the sidelines as the Syrian government and its allies mounted an assault to pin down the rebels in a steadily shrinking pocket of territory in eastern Aleppo, culminating in this week’s ceasefire deal.

“There is absolutely no justification whatsoever for the indiscriminate and savage brutality against civilians shown by the regime and by its Russian and Iranian allies over the past few weeks, or indeed for the past five years,” Kerry told a news briefing in Washington. “We are seeing the unleashing of a sectarian passion.”

He added, “The Assad regime is actually carrying out nothing short of a massacre.”

Kerry said the United States was seeking an immediate, verifiable and durable cessation of hostilities in Aleppo, and said it appeared that air strikes and shelling had stopped and that convoys were moving out.

But there were also reports that a convoy of injured people had been fired on by Syrian government forces or their allies, he said. Activists and residents inside the remaining rebel enclave said this week that pro-government militias had summarily executed dozens of civilians.

Russia has denied that its strikes had killed civilians in large numbers, and said this week that rebels were keeping people in east Aleppo as human shields. Russian Foreign Minister Sergei Lavrov said this month that Russia would treat rebels who stay in east Aleppo as “terrorists.”

LITTLE LEVERAGE

Kerry has repeatedly invested diplomatic capital into deals with Russia that would establish a ceasefire between Assad forces and anti-government rebels, only to have those deals largely ignored by Damascus and Moscow.

In recent days, he has publicly expressed frustration with the complete failure of diplomatic efforts to stop the fighting.

Speaking in the waning weeks of the Obama administration, Kerry on Thursday reiterated long-standing U.S. policy on Syria, and called on the international community to exert pressure on all parties to end the war. Five years of international efforts aimed at reaching a peace deal have failed to do so.

Kerry has little leverage to influence the situation in Syria, former and current diplomats say, partly because of President Barack Obama’s unwillingness to involve the United States heavily in Syria’s war. The United States has provided some support to moderate rebels.

“He just feels that the Russians and the Iranians are culpable and he wanted to make sure that’s clear. At the same time, he also wants to say ‘I am always willing to do more diplomacy,’ which is kind of a ridiculous position,” a U.S. official, who spoke on condition of anonymity, told Reuters.

“It’s not going to get solved in the next (five) weeks. There is something about the Middle East that doesn’t meet U.S. schedules,” the official added, alluding to the Jan. 20 transfer of power from Obama to President-elect Donald Trump.

Obama’s caution, predicated on the goal of avoiding deeper military entanglements in the Middle East, has prompted criticism from officials in his administration, including dozens of American diplomats who wrote a leaked internal memo this year calling for more aggressive action against Assad, including military strikes.

Assad has vowed to fight until he has regained full control of the country. The government’s takeover of Aleppo, the most populous city in Syria before the war, would mark a major victory for him.

Kerry did not elaborate on any influence or leverage the United States could use on Russia to persuade it to pressure Assad to negotiate with members of the Syrian opposition.