United Nations (United States) (AFP) - The United States and Russia failed Friday to renew their pact to impose a ceasefire in Syria after a week of bitter diplomatic battles at the UN General Assembly.

Despite the ferocity of the exchanges and the heavy fighting continuing on the ground, world powers at the meeting agreed the US-Russian talks must continue.

But, as US Secretary of State John Kerry and Russia's Foreign Minister Sergei Lavrov prepared to leave New York, it was clear the sides remained far apart.

The Russian minister said it would be "senseless" to restore a truce because the United States has failed to separate moderate rebel groups from terrorists.

"We're all in favor of the ceasefire, but without the separation of Nusra, or rather the opposition from Nusra, the ceasefire is meaningless," Lavrov declared, referring to the jihadist group the Al-Nusra Front.

Russian-backed Syrian forces ended the week-old ceasefire on Monday and launched an offensive against Aleppo, where US-backed rebels mingle with Al-Nusra members.

The powerful Al-Nusra Front, which rebranded itself as Jabhat Fateh al-Sham in July when it split from the Al-Qaeda movement, is not party to the ceasefire.

"Any truce, seven days, three days, would be senseless," Lavrov said, claiming that "groups close to Al-Nusra" had launched 350 attacks during the week-long ceasefire.

Lavrov also alleged that rebel forces had refused to retreat from the key Castello Road leading into Aleppo, as had been foreseen by the September 9 US-Russian plan.

He complained about Washington's "absolute inability" to make good on its promise to convince the opposition to obey the terms of the truce and separate from Al-Nusra.

"We understand that this is a difficult task, but everything is difficult in Syria," he said.

"We want to see any sign that the coalition has influence on those that are on the ground. I don't think it's asking for much," he added.

And the Russian foreign minister slipped into conspiratorial territory, darkly suggesting the US side might be trying to protect Al-Nusra as a force against Bashar al-Assad's regime.

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"I want to be mistaken," he told reporters innocently.

"But it seems that maybe some people want to spare Nusra and to keep it for a later stage, when the notorious 'Plan B' might be announced."

Earlier this year, Kerry briefed US lawmakers that if negotiations with Russia failed then he would suggest a "Plan B" -- reportedly tougher US military involvement.

Kerry, too, has not minced his words, suggesting this week at the UN Security Council that his long-term sparring partner Lavrov was speaking from a "parallel universe."

But on Friday, after they met for their latest fruitless encounter, he again tried to put an upbeat spin on the dialogue, suggesting there was room for maneuver.

"We have exchanged some ideas," he said. "I think we made a little bit of progress. We're evaluating some mutual ideas in a constructive way, period."

- Historic responsibility -

But the US position has also hardened, with Kerry declaring on Thursday that Moscow must force Assad to ground its air force if the truce is to be revived.

"Let me be clear: The United States makes absolutely no apology for going the extra mile to try to ease the suffering of the Syrian people," he said.

"But we can't be the only ones trying to hold this door open. Russia and the regime must do their part, or this will have no chance," he declared.

French Foreign Minister Jean-Marc Ayrault was not impressed by his colleagues' efforts, sharing his frustration at the failure of their secretive dialogue.

"The American-Russian cooperation has reached its limits. This method is not working. Discussions will continue but they seem interminable," he told reporters.

"The United States has a special responsibility, which has a historic dimension. We ask them to rise to it -- it's time to turn to a more collective approach."

Meanwhile, missiles rained down on Aleppo. According to the Syrian Observatory for Human Rights, 45 civilians were killed on Friday by Russian and regime air raids.