The crisis in Syria — epicenter of Mideast strife and the Mediterranean migration crisis — just got worse. That fact is not only a tragedy for scores of Syrians trapped in a ceaseless cycle of violence that has already killed an estimated 500,000 people, but also a profound problem for world leaders, especially President Obama and soon either Hillary Clinton or Donald Trump.

The most recent deadly escalation came after a rare moment of hope: an ad hoc cease-fire negotiated between the U.S. and Russia that was to allow a week for humanitarian aid to reach war-ravaged Aleppo, as well as to possibly pave a path for joint U.S.-Russian coordination in the ongoing fight against the Islamic State of Iraq and the Levant (ISIL).

But that unraveled fast after renewed spates of violence. First was Saturday’s errant airstrike from the U.S.-led anti-ISIL coalition that instead killed 60 Syrian soldiers. The U.S. military acknowledged the mistake, offered an apology and opened an investigation.

Conversely, it was no mistake, but likely a war crime, when a humanitarian-aid convoy was bombed on Monday. The strike on 18 of 31 aid trucks immediately killed at least 12. And more may die later, since besieged Syrians will be denied the desperately needed cargo of wheat, medicine and clothes. The U.S. military blamed the strike on Syrian and Russian forces, and reportedly there’s evidence that Russian aircraft were responsible for the massacre.

“Just when we think it cannot get any worse, the bar of depravity sinks lower,” said outgoing U.N. Secretary General Ban Ki-moon in his final address to the U.N. General Assembly. “The humanitarians delivering lifesaving aid were heroes,” Ban said. “Those who bombed them are cowards.”

As are those leading them, especially Syrian President Bashar Assad and Russian President Vladimir Putin. Indeed, far from being a strong leader, as Trump has admired Putin, the Russian ruler is a morally weak enabler of an Assad regime that should face multiple war-crimes charges.

At a U.N. Security Council meeting on Wednesday, U.S. Secretary of State John Kerry tried to salvage the cease-fire and called for all military aircraft to be grounded in undetermined “key areas.”

Kerry was unsparing in rejecting Russia’s multiple obfuscations on the convoy attack, saying that Russia was living in a “parallel universe.” And he was uncharacteristically blunt in calling out the Assad regime for using chlorine gas and bombing hospitals in its atrocious, relentless civilian slaughter.

“Kerry knows that you cannot get to first base politically in Syria in the direction of a solution unless you get civilians off the bull’s-eye,” Ambassador Frederic C. Hof, a former special adviser for the transition in Syria who is now a director at the Atlantic Council, told an editorial writer.

And civilians won’t be safe unless responsible nations press Assad and Putin. So despite the odds, Kerry’s dogged diplomacy should continue.