Secretary of State John Kerry
Secretary of State John KerryReuters

U.S. Secretary of State John Kerry warned on Thursday that Washington was “on the brink” of ending its talks with Russia on the Syrian conflict over the assault on Aleppo, AFP reported.

"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 Washington before leaving for Israel as part of the American delegation to the funeral of former President Shimon Peres.

Kerry’s comments came a day after he warned his Russian counterpart Sergei Lavrov he would end talks unless Moscow halts the assault on Aleppo.

Kerry said the United States would pursue other alternatives, "barring some clear indication by the warring parties that they are prepared to consider how to approach this more effectively."

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.

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.

Kerry, however, rejected that criticism, saying on Thursday, "It's easy to be critical of diplomatic efforts because it's difficult, but what is the alternative?"

He said United States would defeat Islamic State (ISIS) jihadists 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."

AFP contributed to this report.