MOSCOW - A Russian defense official said Wednesday a U.S. Predator attack drone showed up nearby just minutes before an aid convoy was attacked while en route to deliver humanitarian relief to rebels in the Syrian civil war.

Russia’s shifting explanations for the attack​ resulted in a war of words at the United Nations, as diplomats from Washington and Moscow traded barbs, and little progress was made in coming to terms with the situation in Syria.

In the immediate aftermath of the attack, aid groups and activists said either Syrian or Russian jets attacked the convoy, something American defense officials have since supported. U.S. officials say a Russian SU-24 attack aircraft was tracked over the aid convoy southwest of Aleppo at the same time -- down to the minute -- that the strike occurred on Monday.

Now, however, Igor Konashenkov, a spokesman for the Russian defense ministry, said Western allegations that Moscow was responsible were an attempt to distract attention from the U.S.-led coalition’s bombing of Syrian soldiers near Deir al-Zor airport​ on Saturday.

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Konashenkov said they have evidence an American attack drone was also in the area, and appeared there just before the attack on the convoy began.

This is at least the third attempt by Russian defense officials at explaining what happened to the convoy. First, the defense ministry said the convoy may not have come under attack at all, and instead just caught fire. Then, the Kremlin released a drone video of its own purporting to show a single rebel vehicle towing a mortar alongside the aid convoy, although Moscow did not say what exactly that meant.

At the United Nations on Wednesday, a clearly angry U.S. Secretary of State John Kerry lashed out over Syrian President Bashar Assad’s backers.

“Supposedly we all want the same goal. I’ve heard that again and again,” Kerry told the Security Council, referencing oft-repeated international objectives of a united, secular and democratic Syria. “But we are proving woefully inadequate in ... making that happen.”

Kerry outlined a litany of U.S. complaints against Assad’s government and its chief backer, Russia. He recited Moscow’s changing narrative over a deadly attack this week on an aid convoy that has included everything from claims of a justifiable counterterror strike to vehicles spontaneously combusting.

“This is not a joke,” Kerry said, sharply criticizing those who engage in “word games” to dodge responsibility over questions of “war and peace, life and death.”

Kerry offered one concrete suggestion to revive diplomatic hopes. Focused on protecting key aid routes in northern Syria, it was unclear if Russia and Syria would agree.

“To restore credibility, we must immediately ground all aircraft flying in those key areas in order to de-escalate the situation and give a chance for humanitarian assistance to flow unimpeded,” he said.

The top American diplomat spoke just after Russian Foreign Minister Sergey Lavrov delivered his own set of barbs, underscoring the breakdown in trust in the 12 days since he and Kerry clinched a cease-fire agreement and a potential U.S.-Russian military partnership against ISIS and al Qaeda. The former Cold War foes and much of the international community hailed the outcome, only to watch it unravel amid an upsurge in violence that even included an accidental U.S. strike that killed more than 60 Syrian soldiers.

Unlike Kerry, who stressed the importance of Assad’s government ending military operations against rebels and allowing in unfettered aid, Lavrov said the U.S. had the biggest responsibility.

“The key priority is to separate the opposition forces from the terrorists,” Lavrov said.

Responding to the wide criticism of the convoy airstrike, which American officials are blaming on Russia, Lavrov cited various possible explanations. Twenty civilians were killed when the Syrian Red Crescent convoy was struck.

Lavrov was more direct in laying out what he presented as a series off truce violations by U.S.-backed rebels groups near the northern city of Aleppo. And he declared Syria’s conflict, as well as that of Iraq, Libya and other instable nations, the “direct consequence” of foreign military interventions in the region. It was a not-so-subtle finger pointed at Washington.

Larvov and Kerry’s speeches laid bare their widely divergent views of a war that has killed up to a half-million people, contributed to Europe’s worst refugee crisis since World War II and allowed the Islamic State to emerge as a global terror threat. At one point, Kerry said listening to his Russian counterpart was like hearing about a “parallel universe.”

On Tuesday, the two diplomats met with more than a dozen Arab and European foreign ministers, hoping to hold onto what might be salvageable from a week of relative calm in Syria. No one spoke of any progress, beyond a decision to hold follow-up discussions later this week.