The Obama administration has found itself increasingly backed into a corner by Russian bombing in Syria that its diplomacy has so far appeared powerless to stop.

Secretary of State John F. Kerry said Friday that he has been in continuous contact with the Russians and that the next few days will determine “whether or not people are serious” about a cease-fire, humanitarian access to areas besieged by fighting and the revival of peace talks suspended this week.

In the meantime, he said, while “civilians, including women and children, [are] being killed in large numbers” and humanitarian access remains denied, the bombing is “not going to stop just by whining about it.”

Earlier this week, Russian Foreign Minister Sergei Lavrov said publicly that his government saw no reason to stop the airstrikes, which Russia says are targeting “terrorist” groups, including those fighting with the Syrian opposition against the government of President Bashar al-Assad.

Vitaly Churkin, Russia’s ambassador to the United Nations, said Friday that accusations that Russia is targeting civilians and opposition forces were in “bad taste.”

A Syrian man comforts a boy amid the rubble of buildings following a reported airstrike on a rebel-held neighborhood in the northern Syrian city of Aleppo on Thursday. (Thaer Mohammed/AFP/Getty Images)

“They’d better put the blame on those who back terrorists,” Churkin said as he entered a Security Council closed-door meeting on Syria.

State Department spokesman John Kirby characterized U.S. efforts to stop the bombing as “assertive diplomacy . . . trying to avert further bloodshed, not escalate the tensions, not make it worse.”

“I recognize that people can argue it’s not going fast enough or we’re not going far enough,” Kirby said. “We’re all frustrated by the situation there. But we also need to be mindful of the ultimate goal here, which is an end to the conflict and not an exacerbation of it.”

The State Department, he said, is “working very, very hard” to ensure that all parties that agreed to a U.N. resolution mandating a cease-fire and peace talks “meet their obligations . . . and that includes Russia.”

Kerry, speaking earlier at a news conference with visiting Colombian President Juan Manuel Santos, indicated that Russia is not the only party at fault.

“Russia has indicated to me very directly they are prepared to do a cease-fire,” Kerry said. “The Iranians confirmed . . . they will support a cease-fire now. We now have to have . . . all the other parties come to the table and acknowledge that they, too, are prepared to do that. And as of this moment, we don’t have that full acknowledgment.”

Kerry is due to meet with Lavrov late next week in Munich, where the 20 members of the International Syria Support Group, including Russia and Assad backer Iran, will hold their fourth meeting to try to forge a cease-fire and a political transition in Syria.

When first scheduled, the Munich meeting was previewed as an opportunity for stakeholders outside Syria to fine-tune peace talks between the government and the Syrian opposition that were supposed to start a week ago. Those talks quickly collapsed, however, amid a ferocious escalation of Russian bombing this week in and around Aleppo that has sent opposition fighters and tens of thousands of new refugees fleeing north toward the Turkish border.

Opposition leaders have accused the Russians of trying to create new facts on the ground to bolster Assad’s negotiating position in anticipation of renewed talks. They have called on the international community, including the United States, to take “concrete steps” to enforce the U.N.-mandated cease-fire.

Kerry said that “a number of modalities for providing . . . humanitarian access” and a cease-fire are being discussed with the Russians. “If it’s just talk for the sake of talk in order to continue the bombing,” he said, “nobody’s going to accept that. And we will know that in the course of the next days.”

What remains unclear is what international refusal to accept Russia’s actions, should they continue, would mean. “I won’t get up here and speculate about options going forward or ruling anything in or out,” Kirby later told reporters. “What we’re focused on is having everybody who signed up to those [U.N.] commitments to meet those commitments, and to stop the suffering.”

Among the humanitarian relief options under consideration by the Obama administration are airdrops to areas where hundreds of thousands of people have been cut off for months from food and medicine and are at risk of starvation.

Administration officials emphasized that airdropped relief is in the planning stages and has not yet been approved. But U.S. forces, which have conducted similar operations in northern Iraq and Syria, could move within days to implement a decision.

In August 2014, the U.S. military airdropped food and water for members of the Yazidi sect, besieged by the Islamic State and stranded on Mount Sinjar in northwestern Iraq. Similarly successful operations were carried out in and around the Syrian town of Kobane, along the Turkish border, where local Kurdish forces were fighting for survival against the militants.

Approval of the airdrop plan, while a noncombat operation, would pave the way for American planes to fly directly into the contested areas of Syria’s civil war for the first time. For the past 18 months, U.S. aircraft have been bombing Islamic State-held areas elsewhere in Syria, but the administration has consistently rejected any air operations related to the separate, civil conflict.

A blizzard of Russian airstrikes in and around the city of Aleppo this week has cut rebel and humanitarian supply lines from Turkey. U.N. efforts to deliver relief on the ground to besieged areas around Aleppo and far beyond have been slowed to a trickle.

The Russian Foreign Ministry said Thursday that Lavrov and Kerry, in telephone conversations this week, “agreed to coordinate possible measures to deliver humanitarian aid to the appropriate districts of Syria by air, using military-transport aircraft.”

U.S. officials, who said the Pentagon has been asked to prepare relief plans, declined to discuss in detail the closely held consideration of the issue. But they said any cooperation with Russia would be only to prevent Russian aircraft from interfering with U.S. operations.

White House press secretary Josh Earnest said Friday that he “wouldn’t rule out humanitarian airdrops in the future.” But, he said, “the amount of supplies that you can include is pretty limited,” and the strong preference is for a cessation of fighting and the lifting of sieges, allowing ground delivery of aid. “You can move a lot more on a convoy of trucks than dropping pallets,” Earnest said.

In the town of Madaya, northwest of Damascus, where a rare convoy arrived last month, dozens of people have reportedly starved to death. “Tragically, there are hundreds of Madayas throughout the country,” U.N. Deputy Secretary General Jan Eliasson said this week. “Humanitarian conditions in besieged and hard-to-reach areas are insufferable and with time, without relief, getting even worse.”

An international donors’ conference, held in London on Thursday, pledged a record $10 billion — including $900 million from the United States — in additional humanitarian aid to Syria and surrounding countries hosting millions of Syrian refugees. But most donors have in the past failed to live up to their commitments.