The United States, Russia and other powers came to an agreement on a “cessation of hostilities” in Syria, but the deal was met with caution and skepticism. (Jason Aldag,Ishaan Tharoor/The Washington Post)

Three days before a planned cease-fire in Syria, there is widespread uncertainty over how it will be implemented and monitored, and what is likely to happen if it does not hold.

Russia and the United States are preparing by Friday to exchange maps delineating their separate assessments of where the Islamic State and al-Qaeda affiliate Jabhat al-Nusra are located in Syria. Both groups are excluded from the cease-fire and an end to Russian bombing.

The two governments will also provide each other with lists of combatant organizations on each side that have agreed to observe what a U.S.-Russia agreement calls a “cessation of hostilities,” senior U.S. officials said.

Many within the Obama administration, particularly in the Pentagon, deeply mistrust Moscow’s intentions. The White House, in a series of high-level meetings in the past two weeks, has requested military and other options in the event the deal collapses.

In Moscow, Russian President Vladimir Putin shakes hand with Syrian President Bashar al-Assad in October 2015. Putin has seized on the Syrian cease-fire deal as a diplomatic victory for Russia. (Alexei Druzhinin/AP)

A defense official said those discussions are still at the “conceptual” stage. “There’s not a whole lot of meat on that bone,” said the official, one of several across national security agencies who spoke on the condition of anonymity about planning. “There is no plan ready for prime time,” another U.S. official said.

Officials cautioned that options under consideration for what Secretary of State John F. Kerry has called a “Plan B” would probably not be directly aimed at punishing Russia but more likely at increased U.S. support for vetted groups of fighters on the ground battling the Russian-backed forces of Syrian President Bashar al-Assad.

That increase, however, is unlikely to include the surface-to-air missiles that are at the top of the rebel wish list. The administration continues to fear that such weapons would end up in the hands of terrorists. There is also concern, administration officials said, that a shoot-down of any of the foreign aircraft flying over Syria — including ones flown by Russia, the United States and Turkey — could spark a far greater international conflagration.

President Obama was noticeably guarded Wednesday about the cease-fire plans. “We are very cautious about raising expectations on this,” he told reporters at the end of an Oval Office meeting with Jordan’s King Abdullah II. When the king said he had his “fingers crossed,” Obama interjected, “We’ll see.”

Obama expressed limited expectations for the near future, speaking of “some lessening of the violence” over the next several weeks “that provides us a basis to build a longer-term cease-fire” and hopefully a political transition that will end Syria’s civil war.

Even Kerry, who negotiated the deal early this month with Russian Foreign Minister Sergei Lavrov, told lawmakers Wednesday that “I’m not vouching for the fact that this cease-fire will absolutely work and take place.”

After a Tuesday hearing in which Kerry told the Senate Foreign Relations Committee that “significant discussion” was underway over a Plan B, Sen. Bob Corker (R-Tenn.), chairman of the panel, added his name to the list of skeptics.

“Show me,” Corker said in an interview. “I don’t believe Russia believes there’s a Plan B. I don’t believe Assad and Iran believe there’s a Plan B. . . . I hope, candidly, that Russia will agree and they will cease activities. But at this point, I don’t believe they believe there’s any price to pay if they don’t.”

For the moment, all sides insist they are focusing on Plan A. Both the Assad government and an opposition umbrella group representing some, but not all, rebel groups on the ground have agreed to the cease-fire.

Once it goes into effect, at midnight Friday, Damascus time, anyone claiming a violation will be invited to call, email, text or otherwise contact an adjudication center currently being set up by a U.S.-Russia-led task force of outside stakeholders in the Syrian war, along with a U.S.-Russia hotline. But it remains unclear how Washington or Moscow will determine the truth on a battlefield where neither has forces present, or how they will impose a ruling.

“We’re not against it,” Salman al-Meslet, spokesman for Syrian opposition leaders meeting Wednesday in Riyadh, Saudi Arabia, said of the cease-fire plan in a telephone interview. “We really want an end to this bloodshed.”

But “is there any guarantee that Russia will not violate” the agreement and continue bombing the opposition, as it has under the guise of striking terrorist groups such as Jabhat al-Nusra? Meslet asked. “What action will the [United] States take? What will they do to stop them?”

“We want assurances from the States, from the Europeans, from the Saudis and others,” he said. “It’s not a condition from our side, but it’s a question we want answers to.”

The cease-fire and an agreement by all sides to allow unhindered humanitarian aid to besieged communities inside Syria are supposed to lead to negotiations between the Assad government and the opposition toward establishing a transitional government. U.N. envoy Staffan de Mistura hopes to convene the talks in Geneva on Monday or Tuesday, U.S. officials said.

Sieges were lifted to allow assistance to reach five Syrian towns last week — and Russian planes dropped World Food Program pallets for another town Wednesday — but the aid program has been slow to get off the ground.

A wild card in the cease-fire equation is Turkey, a U.S. ally, NATO member and leading player in the Syria drama. President Recep Tayyip Erdogan said in a speech in Ankara on Wednesday that the Syrian Kurdish group that has received U.S. support for its fight against the Islamic State should be excluded from any truce, allowing the Turkish military to continue cross-border shelling against the group.

Turkey has been urging the United States to sever ties with the Syrian Kurdish People’s Protection Units (YPG) on grounds that it has ties to the Kurdistan Workers’ Party (PKK), which is designated a terrorist organization by both Turkey and the United States.

“Our allies must understand that they are at a crossroads,” Erdogan said of the United States, saying that its insistence that the YPG should not be deemed terrorist is “an insult to our intelligence and the intelligence of the whole world.”

Erdogan also questioned U.S. assertions that the YPG’s role in fighting the Islamic State makes it an indispensable ally, pointing out that Jabhat al-Nusra also battles the Islamic State.

Meanwhile, Russian President Vladi­mir Putin worked to place himself in the center of diplomatic efforts to secure the cease-fire, consulting by phone with the leaders of his allies Syria and Iran, and also King Salman in Saudi Arabia and Israeli Prime Minister Benjamin Netanyahu, according to the Kremlin.

Putin has seized on the cease-fire deal as a diplomatic victory for Russia, placing Moscow on the same superpower level as the United States, long a Kremlin goal. His spokesman, Dmitry Peshkov, said that U.S.-Russian “joint actions” on Syria were leading to a “higher level of confidence” between the two, according to the Russian news service Interfax.

Such talk is said to infuriate Defense Secretary Ashton B. Carter. “He doesn’t want to play ball with the Russians at all, no question about that,” one senior administration official said.

Michael Birnbaum in Moscow, Hugh Naylor in Beirut and Missy Ryan and Karoun Demirjian in Washington contributed to this report.