The Obama administration threatened Wednesday to suspend plans for coordinating counter­terrorism strikes in Syria with Russia unless Moscow moves to stop its assault on the city of Aleppo and restore a collapsed cease-fire.

Secretary of State John F. Kerry, in a telephone call to his Russian counterpart, “made clear the United States and its partners hold Russia responsible for this situation,” including the “drastic escalation” of its military actions in recent days and its refusal to ensure the safe passage of humanitarian aid to government-besieged areas, the State Department said.

The United States, Kerry told Foreign Minister Sergei Lavrov, “is making preparations to suspend U.S.-Russia bilateral engagement on Syria . . . unless Russia takes immediate steps to end” the heightened carnage in Aleppo, where at least a quarter-million civilians are trapped amid withering Syrian and Russian bombing.

The scale of the slaughter is unprecedented in Syria’s five years of civil war, as are the international cries for something to be done. Yet there is a growing sense that the United States is powerless to stop it, or is at least unwilling to take steps that might force Moscow and Damascus to change their calculations.

Those possible steps — including U.S. military action, increased arms shipments to Syrian rebel forces or additional sanctions — have changed little over the years of Syria’s civil war.

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All are again being reviewed by the administration, including at a National Security Council deputies meeting Wednesday to prepare options for top national-security officials.

But just as the alternatives are the same, so are the perceived drawbacks and internal opposition to each of them. If anything, the fast-approaching end of the administration and the complications of the conflict — which now involves a bewildering mix of rebels, terrorists and the forces of several governments — have made them even less attractive, according to several senior administration officials who spoke on the condition of anonymity about the closed-door discussions.

Kerry and President Obama believe that none of the options “are better, in the long run, for the Syrian people than trying to get a diplomatic solution now,” State Department spokesman John Kirby said. Kirby and other officials declined to define “immediate” in the context of Wednesday’s demand for Russian action and emphasized that the door, for now, remains open to Moscow.

In Moscow, Lt. Gen. Viktor Poznikhir of the Armed Forces General Staff did not mention the U.S. threat. Following the Kerry-Lavrov call, he said that, “at the behest of the Russian president,” Russia would “further continue the joint work with our U.S. partners on the Syrian problem,” according to Interfax, the Russian news agency.

The threat of suspending “bilateral engagement” largely refers to ongoing U.S.-Russia talks in Geneva over plans for coordinating strikes against al-Qaeda and the Islamic State. Under a plan agreed to by Kerry and Lavrov this month following lengthy diplomatic negotiations, the cooperation was to follow seven consecutive days of “reduced violence” and the distribution of humanitarian aid.

Russia agreed to ground Syria’s air force, while the United States agreed to separate the opposition forces it supports from the terrorist groups with which they are increasingly intertwined.

The truce, which began Sept. 12, lasted only a few days before fighting began again in and around the divided city of Aleppo, with each side blaming the other with starting it. An apparently mistaken U.S. airstrike that killed Syrian troops in eastern Aleppo, followed by a strike against a humanitarian convoy outside Aleppo for which the United States blamed Russia, significantly raised temperatures.

Warplanes knock out Aleppo hospital as a Russian-backed assault intensifies. (Reuters)

Late last week, the government of Syrian President Bashar al-Assad declared the truce at an end and began a new offensive intended to retake eastern Aleppo from the now-surrounded rebels.

Amid continued Syrian and Russian airstrikes pummeling the ravaged city, using incendiary and bunker-busting bombs that reach even the underground caverns where schools and hospitals have operated for years, critics were quick to belittle the administration’s response.

Senate Armed Services Committee Chairman John McCain (R-Ariz) and Sen. Lindsey O. Graham (R-S.C.), who last week called Kerry “delusional” for trusting Russia, sarcastically labeled the threat of suspending cooperation “a real power move in American diplomacy. . . . We can only imagine that having heard the news, [Russian President] Vladi­mir Putin has called off his bear hunt and is rushing back to the Kremlin to call off Russian airstrikes on hospitals, schools, and humanitarian aid convoys around Aleppo,” they said in a Wednesday statement.

Kerry has been unrepentant in trying to take diplomacy to its outer limits. “Where’s the congressional vote for force?” he said Monday, referring to congressional refusal, in 2013, to back Obama’s plans to punish Assad with airstrikes for his use of chemical weapons. “Talk is cheap,” Kerry said. “I have my own views about what we ought to do, but I’m not going to be arguing about them publicly.”

“The cause of what is happening is Assad and Russia wanting to simply try to pursue a military victory. And it would be diplomatic malpractice not to try to pursue whether or not through some kind of diplomatic effort you could actually wind up reducing the violence,” he said. “And what’s the alternative? Today there’s no cease-fire, and we’re not talking to [Russia] right now. What’s happening? The place is being utterly destroyed. Okay?”

“That’s not delusional,” Kerry said. “That’s a fact.”

Long an internal proponent of a more muscular U.S. response to Assad, including the use of U.S. military power, Kerry has largely kept his peace since Russia first sent its air force to Assad’s aid last fall. For its part, the Pentagon has not wavered from a belief that entering the civil war against Assad would detract from the separate fight against the Islamic State in Syria and Iraq and risk a broader conflagration. Although Obama has overruled Defense Department objections to cooperating with Russia and supported Kerry’s efforts, he has long been unwilling to approve any Plan B for what to do if diplomacy or limited efforts to arm the opposition fail.

While still on the table, military options such as bombing Assad’s airstrips, once the subject of lengthy internal U.S. discussions as to legality and effectiveness, now seem impossible with Russia’s participation in the air war.

Some officials have continued to advocate allowing more-eager backers of the rebels, including Saudi Arabia and Qatar, to supply them with more-sophisticated weapons, including shoulder-launched antiaircraft missiles. But long-standing U.S. fears that the weapons could fall into terrorist hands now seem even more acute, with the increased mixing on the ground of rebel groups and the potent forces of the newly renamed Front for the Conquest of Syria, the former al-Qaeda affiliate known as Jabhat al-Nusra.

“I just don’t see it at the moment,” a Western official, whose government is part of the anti-Islamic State coalition, said of any U.S. military steps. This White House, the official said, “hasn’t got much longer, and there’s a reluctance to do anything that would bounce a future administration into a course of action.”

French Foreign Minister Jean-Marc Ayrault on Wednesday proposed a new U.N. Security Council resolution for an Aleppo cease-fire, saying “those who don’t vote for it risk being held responsible for complicity in war crimes.”

The goal, the Western official said, is “to shame Russia. . . . But Russia has shown that it isn’t easily shamed.”

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