"I think we are on the verge of suspending the discussion because, you know, it's irrational in the context of the kind of bombing taking place, to be sitting there, trying to take things seriously," Kerry said.
"There is no notion or indication of seriousness of purpose with what is taking place right now," he told a conference in the US capital, a day after warning his Russian counterpart Sergei Lavrov he would end talks unless Moscow halts the assault on Aleppo.
A Kremlin spokesman, Dmitry Peskov, said Moscow would press on with its bombing campaign in Syria, where forces loyal to President Bashar al-Assad are waging a furious assault on Aleppo's rebel-held eastern sector.
A top UN aid official called the situation in Aleppo a "merciless abyss of humanitarian catastrophe," in the latest appeal for a halt to fighting.
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Kerry's efforts to broker a ceasefire have come under fire from Republicans who have called for tougher action against Moscow and the Syrian regime.
He said United States would defeat the Islamic State militants operating in Syria and Iraq, "but that is different and distinct from involving ourselves directly into the civil war which is the war against Assad."
His comments echoed those of President Barack Obama who insisted on Wednesday that "there is not a scenario in which, absent us deploying large numbers of troops, we can stop a civil war in which both sides are deeply dug in.
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