A cease-fire in Syria brokered by the United States and Russia has brought a rare period of calm to hard-hit parts of the country, activists said Tuesday, sending normally terrified residents out to shop, play and visit loved ones.

The nationwide truce went into effect at sundown Monday and continued to hold despite reports of sporadic violations.

After some airstrikes and shelling in the early hours, “by early morning every report we have been seeing indicates a significant, significant drop in violence,” U.N. envoy Staffan de Mistura said Tuesday evening in Geneva.

It is the second major attempt by world powers to curb the fighting that has killed nearly half a million people and spawned a humanitarian crisis stretching from the Middle East to Europe. It also marks the first time in months that rebel forces and the Syrian government have halted their fire in all opposition areas, including government airstrikes, granting civilians a brief reprieve from the constant threat of bombing and shelling.

“People are going about their business, and children are even playing in the street,” Mohamed Omar, a civil defense volunteer in the rebel-held area of Aleppo, a city that has been devastated by the five-year conflict.

The United States and Russia announced a new multi-step plan to bring Syria closer to a negotiated peace deal. (Jason Aldag,Karen DeYoung/The Washington Post)

“People feel a lot more safe today because there are no airstrikes,” he said. “Things seem calmer.”

The cease-fire is part of a broader deal between Washington and Moscow to eventually launch joint strikes against Islamist militants in Syria and possibly pave the way for a negotiated end to the war. The United States has backed some Syrian rebel factions battling the government and is leading a coalition conducting strikes against the Islamic State in Syria. Russia has sent warplanes and troops to aid Syrian President Bashar ­al-Assad.

The two sides also have called for the unimpeded flow of aid into Syria. But that effort had yet to get off the ground as of late Tuesday.

A major road designated under the agreement as a neutral humanitarian corridor remained the scene of clashes. Syrian government forces are required by the deal to withdraw from Castello Road in Aleppo province, and rebels also should refrain from launching attacks.

However, in a Russian Defense Ministry broadcast aired on the state-funded RT network, two Russian military commanders are shown ducking for cover as their position comes under fire. De Mistura said that allegations of mortar attacks on the road, “by the opposition, in this case,” had been reported.

De Mistura said he expected aid convoys to begin moving “very soon.” The trucks destined for ­rebel-held eastern Aleppo, he said, will move once “notification” of their contents is formally provided to the Syrian government and opposition forces in the city formally agree to U.N. distribution terms.

Aid destined for other parts of the country, where more than a dozen besieged areas have been without food deliveries, in some cases for years, is still awaiting Syrian government authorization letters, de Mistura said.

He indicated that, under the terms of the U.S.-Russia agreement, the aid should begin to flow within the first 48 hours of the cease-fire.

The United Nations needs to make sure its staff and partners “are not in mortal danger” before starting aid convoys, said Jens Laerke, a spokesman for the U.N. coordination agency in Geneva, the Associated Press reported.

But whether the deal can keep violence in check remains to be seen.

According to the timetable laid out by Secretary of State John F. Kerry and Russian Foreign Minister Sergei Lavrov, the first seven days are crucial. Under the deal, Russia is responsible for keeping the Syrian air force out of the air, while the United States must stop attacks by rebel forces.

“The first step is a cessation of hostilities for seven days. And this is something the Russians and the regime must do, and they must do it properly,” Lt. Gen. Jeffrey Harrigian, head of U.S. Air Forces Central Command, told reporters in a video briefing.

If the cease-fire holds, the two sides will start working out plans to share intelligence and conduct coordinated airstrikes, first against a former al-Qaeda affiliate, Jabhat Fatah al-Sham, or Front for the Conquest of Syria, and eventually against the Islamic State.

But a number of rebel groups have been fighting side by side with Jabhat Fatah al-Sham, raising concerns among opposition members that other factions will be targeted as well. The administration has acknowledged the overlap between the groups and warned the rebels that they must separate themselves from the terrorists or risk falling under coordinated U.S. and Russian strikes against the militants once they begin.

“We’re not being unrealistic,” said a senior administration official, speaking on the condition of anonymity to provide more details. “It’s not going to happen overnight. It’s certainly not going to happen” if Russia and Syria don’t stop “indiscriminate” bombing. But “at some point,” the official said, “it is going to be important for the opposition to distance themselves.”

Rebel fighters also have criticized the agreement for not imposing direct penalties for violations, and they say it could give Assad’s military a chance to try to expand its grip.

Rebel neighborhoods in eastern Aleppo have been “de facto under siege” by Syrian and Russian airstrikes, with 275,000 residents almost “entirely cut off from vital supplies” such as food, water and electricity, the United Nations has said.

But on Tuesday, residents there and in nearby Idlib breathed a sigh of relief — even if just temporarily.

Mohib Abdelsalam, a 26-year-old emergency responder and member of the Aleppo Revolutionaries rebel group, said residents had hoped that aid would arrive Tuesday.

“We are trying to remain hopeful,” Abdelsalam said. “We need petrol and food, fresh produce. We have been depending mostly on vegetables that we grow ourselves. We are currently just trying to survive the day.”

In Kafranbel in Idlib province, also in the north, Abu Muhammad said residents are relieved that no airstrikes have occurred there.

“We haven’t heard any planes in the sky, and we haven’t documented any [cease-fire] violations in our area,” said Muhammad, an activist and lawyer.

But people “are skeptical about whether it is going to work,” he said.

Heba Habib in Berlin, Zakaria Zakaria in Istanbul and Missy Ryan in Washington contributed to this report.