A boy walks amid damaged buildings in a rebel-held area of Aleppo, Syria, on Sept. 29. (Abdalrhman Ismail/Reuters)

Russia dismissed U.S. threats to suspend plans for bilateral cooperation on counterterrorism operations in Syria and instead charged the Obama administration with making “groundless accusations” against Moscow to “camouflage” its own inability to comply with their Syrian cease-fire agreement.

“Verbal attacks against our country and absolutely groundless accusations of all mortal sins against Russia,” Kremlin spokesman Dmitry Peskov said Thursday, “ . . . disagrees with reality.”

Moscow’s response came a day after the administration warned that it would not continue with plans to implement their cooperation agreement unless Russia and Syria immediately stopped their bombing campaign in Aleppo.

Russia said it would consider a 48-hour truce to allow delivery of humanitarian aid to surrounded, rebel-occupied eastern Aleppo. But Deputy Foreign Minister Sergei Ryabkov said the seven-day cease-fire it initially agreed to with Washington was now “unacceptable” because it would allow “terrorist groups to take necessary measures to replenish supplies, regroup forces.”

Moscow has cited Washington’s inability to separate opposition forces it supports from al-Qaeda-allied terrorists fighting alongside them as a violation of the cease-fire agreement they announced Sept. 9. Washington has said that Russia and the Syrian government of President Bashar al-Assad are violating the agreement — and preventing the separation of forces — by continuing airstrikes.

The United Nations indicated that 48 hours was the minimum time required to deliver aid once the Assad government issues safe-passage permits. At the same time, hundreds of civilian wounded cannot be treated — with hospitals bombed and doctors killed in massive airstrikes over the past several days — and need urgent evacuation, the deputy U.N. envoy for Syria, Ramzy Ezzeldin Ramzy said in Geneva.

In Washington, Secretary of State John F. Kerry, who negotiated the agreement with Russian Foreign Minister Sergei Lavrov, said the ongoing Aleppo bombing was “beyond the pale of any notion, strategic or otherwise,” and “completely against the laws of war.”

Kerry, speaking at a forum sponsored by the Atlantic magazine and the Aspen Institute, said that “we’re on the verge of suspending the discussion” with Russia. “It’s one of those moments where we’re going to have to pursue other alternatives for a period of time,” he said, “barring some clearer indication by the warring parties that they’re prepared to consider how to approach this more effectively.”

The White House declined Thursday to specify what those “other alternatives” are.

“There are a variety of contingency plans that the president’s national security team is always considering,” White House press secretary Josh Earnest said. “In some cases, we won’t discuss them . . . because we’re invested in trying to make the original plan work.” In others, he said, the options “haven’t been cleared for public discussion.”

Administration officials have said that options under consideration include allowing regional partners such as Saudi Arabia and other countries to step up their weapons aid to anti-Assad opposition fighters and increasing sanctions against Syria and Russia. President Obama has long believed that U.S. military intervention in the Syrian civil war would only make the conflict worse while taking resources from his top priority, the separate fight against the Islamic State in Syria and Iraq.

In a contentious hearing on Capitol Hill, lawmakers charged that the administration has no “Plan B” to use as leverage with Russia in the event diplomacy fails.

“There’s no support from the White House” for Kerry’s diplomatic efforts, Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.), told Kerry’s deputy, Antony Blinken.

Calling Kerry a “sympathetic figure,” Corker said the secretary had been left hanging by a White House unwilling to “roll up sleeves and deal with the tough issues that we have to deal with. . . . Diplomatic actions cannot be backed up, because Russia and Assad realized that there is no Plan B, never has been a Plan B.”

“I think on all of you guys who write books after you leave,’ Corker told Blinken. “I think it’s going to be a fascinating walk through what I believe to be a failed presidency as it related to foreign policy.”

Sen. Benjamin L. Cardin (Md.), the ranking Democrat on the committee, congratulated Kerry on negotiating the agreement,but said that “now, we are clearly at an inflection point.”

“The U.S.-Russia cease-fire agreement was based on the assumption that Russia could compel the Assad regime to ground its air force, that Russia would compel the Assad regime to allow immediate and unfettered humanitarian access,” Cardin said. “We have clearly seen that neither of these two objectives were achieved.”

Now, he said, “I look forward to hearing from you, Mr. Blinken, on what actions the United States is considering, what are our options and how can Congress be your partner.”

When it was Corker’s turn again, he said, “I’ve never seen signs of a Plan B.”

Blinken embarked in a different direction. “How do civil wars end?” he asked rhetorically. “And we know from history and experience —”

“I don’t want a history lesson,” Corker interrupted. “I’d like to understand what Plan B is.”