UNITED NATIONS/BEIRUT: U.S. Secretary of State John Kerry Wednesday demanded that Russia and the Syrian government immediately halt flights over Syrian battle zones, in what he called a last chance to salvage a collapsing cease-fire and find a way “out of the carnage.”

An impassioned Kerry faced off with Russia’s Foreign Minister Sergey Lavrov at the U.N. Security Council in New York, in an unusually heated televised showdown, saying the bombing of an aid convoy in Syria raised “profound doubt whether Russia and the Assad regime can or will live up to” cessation of hostilities obligations.

Listening to Lavrov made him feel like he was living in a “parallel universe,” Kerry said.

On the ground, rebels battled against the forces of the government of President Bashar Assad on major front lines near Aleppo and Hama, and airstrikes reportedly killed a dozen people, including four medical workers.

“I emphasize this to Russia. The United States continues to believe there is a way forward that, although rocky and difficult and uncertain, can provide the most viable path out of the carnage,” Kerry said.

“If we allow spoilers to choose the path for us, the path of escalation ... then make no mistake my friends: The next time we convene here, we’re going to be facing a Middle East with even more refugees, with more dead, with more displaced, with more extremists and more suffering on an even greater scale.”

He mocked what he described as absurd Russian explanations for an attack on an aid convoy Monday that Washington says was carried out by Russian warplanes. A Russian statement said the trucks had “caught fire,” which Kerry called tantamount to blaming “spontaneous combustion.”

“To restore credibility to the process, we must move forward to try to 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.

Lavrov, for his part, called for an independent investigation into the convoy attack, and said all parties needed to take simultaneous steps to stop the war.

U.N. officials said Wednesday the United Nations was gearing up to resume aid deliveries in Syria suspended after the deadly attack.

The cessation agreement, which took effect last week, is probably the last hope for a settlement on Syria before the administration of President Barack Obama leaves office, and has been Kerry’s main focus for months. But it has so far followed the path of all previous peace efforts: Abandoned by the warring parties even as diplomats far away debate it.

Kerry said it was a moment of truth for the opposition, which he said had to do more to distance itself from the Nusra Front, long Al-Qaeda’s Syrian wing, which changed its name to Jabhat Fatah al-Sham and disavowed Al-Qaeda two months ago.Nusra is still characterized by both the West and Moscow as a terrorist group excluded from the cessation of hostilities. Other rebels say Moscow and Damascus use this to justify broader attacks.

Overnight fighting was focused in areas that control access to Aleppo city, where the rebel-held east has been besieged by Russian and Iranian-backed pro-government forces and completely encircled for all but a few weeks since July.

Syrian state media said the army had recaptured a fertilizer factory in the Ramouseh area to the southwest of the city. The Britain-based Syrian Observatory for Human Rights, an activist group, confirmed the advance and said government forces had pressed forward near an apartment complex nearby.

A rebel fighter in the Aleppo area said warplanes had been bombing all night in preparation for an attack. But “the regime’s attempts to advance failed,” said the rebel, speaking to Reuters via the internet.

A Syrian military source said insurgent groups were mobilizing to the south and west of Aleppo and in the northern Hama area. “We will certainly target all these gatherings and mobilizations they are conducting.”

The army reported carrying out airstrikes on seven areas near Aleppo. The Observatory said that one airstrike killed four medical workers and at least nine rebel fighters in the insurgent-held town of Khan Touman south of Aleppo.

The medical staff killed were working for the Union of Medical Care and Relief Organizations, it said. UOSSM confirmed in a statement that at least four of its staff had been killed.

Syrian government forces also launched a major advance in Hama province in the west of the country.

“It is a very intense attack, for which Russian jets paved the way, but it was repelled by the brothers, praise God,” Abu al-Baraa al-Hamawi, a rebel commander fighting as part of the Islamist Army of Conquest, told Reuters.

He said rebels had destroyed four tanks and inflicted heavy losses. Syrian state TV said government forces had killed a number of insurgents and destroyed their vehicles.

Rebel sources also reported an attempt by pro-government forces to advance in the Handarat area to the north of Aleppo, saying this too had been repelled. Pro-government media made no mention of that attack.

The Observatory reported that a Syrian jet had crashed near Damascus, saying the cause of the crash and fate of the pilot were unknown. Daesh (ISIS) said it had been shot down.