On Monday, John Kerry said it would be "diplomatic malpractice" to abandon talks with Russia on restoring a ceasefire in Syria. On Wednesday, he threatened to do just that.

In a call to his partner in a year-long diplomatic dance, Foreign Minister Sergei Lavrov, the US secretary of state demanded that Russia halt the Syrian regime's assault on Aleppo.

"He informed the foreign minister that the United States is making preparations to suspend US-Russia bilateral engagement on Syria," Kerry's spokesman John Kirby said.

If Russia does not force Syrian leader Bashar al-Assad to halt his attack on rebel-held eastern Aleppo, Washington will cancel a plan to open a joint US-Russia military targeting cell, he said.

Last week, Lavrov declared calls for a new ceasefire "senseless." So it seems unlikely Moscow will back down now, leaving the diplomatic track blocked and a quarter-of-a-million Syrians under bombardment.

Kerry's spokesman said the secretary warned Lavrov that Russia would now be held responsible for the situation, including "the use of incendiary and bunker buster bombs in an urban environment."

But just a day before Kerry's ultimatum, the State Department was defending his efforts to keep the door open to Moscow.

Under skeptical questioning from reporters, spokesman Mark Toner said "really, until the past few weeks, we felt like we were on a firm path towards a possible diplomatic resolution to this."

This optimism has attracted much criticism, even scorn, in Washington, where experts have lined up to accuse Kerry and President Barack Obama of being fooled by Moscow.

The State Department's former special adviser on Syria, Fred Hof, was scathing, mocking the administration's faith that Russia's demand for military cooperation represented a diplomatic opening.

"This is the sad, pointless diplomacy of desperation and wishful thinking," he wrote on the blog of think tank the Atlantic Council.

For Hof, Kerry's mission was a "fool's errand", but mainly because Obama gave him no tools to pursue any other option -- no leverage to convince Vladimir Putin that military victory is impossible.

It has become a mantra of US officials -- now repeated by Lavrov and UN peace envoy Staffan de Mistura -- throughout the process that "there is no military solution" to the crisis in Syria.

But this has not stopped Obama's critics from proposing them.

"A careful consideration of military options is not pleasant work for any American president. Yet in this case it must be done," wrote Hof, arguing that Assad's brutality must exact a price.

Washington's Arab allies, led by Saudi Arabia, have demanded that opposition rebels fighting Assad be given advanced weapons such as MANPADS -- shoulder-fired anti-aircraft missiles.

This might alter the balance of power on the battlefield and make Moscow and Damascus more ready to talk. But US officials worry that the arms would leak from moderate groups to jihadist extremists.

Turkey has called for a no-fly zone and for US assistance to carve out a buffer zone in northern Syria to shelter rebels and refugees, but Obama has been wary of getting US troops drawn into the war.

Tougher sanctions may be applied on Syrian and Russian officials, but many in Putin's inner circle are already blacklisted for their roles in the annexation of Crimea, with no change in his stance.

Besides, Washington has opposed new sanctions in the past in order to avoid derailing the outreach to Russia. Might that change now?

UN Secretary-General Ban Ki-moon accuses those who are bombarding Aleppo's hospitals and schools of war crimes, and there have been calls for new international sanctions.

On Tuesday, Obama's spokesman Josh Earnest said "we've not taken the prospect of additional financial sanctions off the table" but said these would have to be coordinated internationally.

In the meantime, Putin and Assad remain bent on pursuing a military solution -- and the victim is not just the Syrian people but for many in Washington the credibility of US leadership.