A day after a recording leaked of secretary of state John Kerry accusing the Kremlin of disrespecting international law, Russia’s foreign ministry warned the US not to cause “terrible tectonic shifts” with strikes on Syrian government forces.

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On Friday, audio of Kerry saying he had “lost the argument” for the use of force against the Syrian president, Bashar al-Assad, was published by the New York Times. The recording was reportedly from a private meeting with non-Syrian diplomats and Syrian civilians who work for aid groups.

“I think you’re looking at three people, four people in the [Obama] administration who have all argued for use of force, and I lost the argument,” Kerry said. “I’ve argued for the use of force.”

News of Kerry’s comments followed the collapse this week of an agreement brokered by the secretary of state and his Russian counterpart, Sergei Lavrov, to create a ceasefire in the five-year civil war between forces loyal to Assad, anti-government rebels and militant groups such as Islamic State.

Russia, which has continued to bomb the city of Aleppo, on Saturday accused the US of failing to live up to its side of the bargain.

“If the US backs out of agreements or refuses to execute them, it’s another illustration and piece of evidence that they are playing into the hands of extremists and terrorists,” ministry spokeswoman Maria Zakharova told Russian television on Saturday, according to the state-owned media outlet Ria Novosti.

The Kremlin has categorized all rebels opposed to Assad as terrorist groups, and for months waged an indiscriminate bombing campaign on various cities, despite the presence of large civilian populations. A joint Syrian government-Russian campaign in Aleppo, where some 250,000 people are trapped, has over the last few days killed 338 people, including 100 children, according to the World Health Organization.

United Nations secretary general Ban Ki-moon said this week that attacks on hospitals amounted to a war crimes.

Earlier in September, warplanes from the US-led coalition against Islamic State struck Assad’s forces while attempting to attack militants, prompting an emergency meeting of the UN security council. Coalition air strikes against Assad, Zakharova warned on Saturday, could create destabilizing ripples across the Middle East.

“If the US begins direct aggression toward Damascus and the Syrian army, it would lead to terrible tectonic shifts not only in the territory of this country, but also, in principle, in the region,” she said.

“More than this,” Zakharova added, “here and there there have been leaks of discussions of possible scenarios” that would have “a powerful effect on Damascus, which is to say, in the large part, direct aggression”.

In the leaked audio, Kerry spoke with frustration about resistance within the Obama administration and US public to military action in Syria, saying: “The bottom line is that we, the Congress refused even to vote to allow that.”

But Kerry blamed the failure of the ceasefire deal on Russia, saying the US had no legal grounds to attack Assad. “The problem is the Russians don’t care about international law and we do,” he said.

“And we don’t have a basis, our lawyers, unless we have a UN security council resolution, which the Russians can veto and the Chinese, or unless we are under attack from the folks there, or unless we are invited in. Russia is invited in by the legitimate regime – well, it’s illegitimate in my mind – but by the regime.”

Kerry also warned his audience that the US did not want the situation to spiral yet further out of control and even suggested the possibility of elections, including Assad, as part of a political transition the US hopes would remove the president from power.

“The problem is,” he said, “you get, quote, ‘enforcers’ in there and then everybody ups the ante, right? Russia puts in more, Iran puts in more; Hezbollah is there more and Nusra is more; and Saudi Arabia and Turkey put all their surrogate money in, and you all are destroyed.”

On Friday, state department spokesman Mark Toner told reporters diplomacy in Syria was “on life support, but it’s not flatlined yet”. “If we do walk away from this diplomatic process,” he said, as “moribund as it is, what are the options?”

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As of February, before Russia intensified its bombing campaign, more than 470,000 people had been killed in the civil war in Syria, according to the Syrian Center for Policy Research.

Earlier this week, ambassadors from the US, UK and France walked out of the UN in New York in protest when a representative for Assad moved to speak. Russia has found itself increasingly isolated even as its influence in Syria has grown.

Sweden’s foreign minister, Margot Wallstrom, criticized both governments on Saturday, writing on Twitter: “Unacceptable to bomb civilians, children and hospitals in Aleppo. No humanity. Assad & Russia moving further away from peace.”

US-led strikes against Isis have continued unabated. Syrian state media and Deir el-Zour 24, an activist media collective, reported on Saturday that coalition warplanes had destroyed several bridges over the Euphrates in a region held by the terror group.