Russia and the United States have traded angry accusations over the attack of a Syrian humanitarian aid convoy, as U.S. Secretary of State John Kerry called for a no-fly zone in some parts of northern Syria.
The White House has openly blamed Moscow for the September 19 attack that killed dozens and destroyed aid being delivered to a town near besieged Aleppo.
U.S. officials have said privately that two Russian Su-24 jets were in the skies directly above the convoy at the time of the attack.
Moscow has angrily rejected those accusations, offering evidence it says shows a U.S. drone in the vicinity of the attack. Moscow's accusations echoed earlier denunciations of a U.S. air strike on a group of Syrian soldiers over the weekend.
Washington has denied any U.S. drones were in the area, and said that the earlier attack was a mistake.
At the UN Security Council on September 21, Kerry said a no-fly zone could restore credibility to a fraying cease-fire and allow humanitarian aid to flow into people living in besieged areas.
Kerry said it was imperative that Russia force the Syrian government of President Bashar al-Assad to ground its planes.
Russian Foreign Minister Sergei Lavrov said at the Security Council meeting that Russia and Syria would not take part in any more "unilateral pauses." He said opposition groups fighting the Syrian regime used them to rearm themselves and strengthen their positions.
Lavrov said the United States bore the biggest responsibility for peace by separating opposition forces from terrorists.
He called for the United Nations to expand its terrorism list to include groups at the fringes of a U.S.-backed rebel umbrella group and repeated earlier denunciations of the U.S. strike on Syrian positions as an "outrageous violation."
The United Nations, meanwhile, said late on September 21 that it was preparing to resume aid deliveries that had been suspended after the strike.
But underscoring the unremitting violence against aid groups, a relief aid group reported on September 21 that an air strike in northern Syria killed four medics responding to an earlier bombing raid.
The attack that killed the medics took place shortly after they arrived at the scene of an air strike in the rebel-held town of Khan Touman.
As the medics deployed, planes circled around and struck the area again, said Oubaida al-Moufti, vice president of the International Union of Medical Care and Relief Organizations.
The group later said two nurses and two ambulance drivers were killed, while a third nurse was in critical condition.
The phenomenon of having warplanes return immediately to the site of an earlier attack and hit it again is known as a "double tap." Syrian government forces have been accused of carrying out "double tap" attacks throughout the 5 1/2-year war, placing paramedics and rescue workers in peril.
After the Security Council meeting, Kerry summed up the situation in a message posted to Twitter:
“This is a moment of truth in Syria, for Russia & the regime, for opposition, for international community. Cannot let spoilers prevail," he wrote.