Syrian men carrying babies make their way through the rubble of destroyed buildings following a reported airstrike on the rebel-held Salihin neighborhood of the northern city of Aleppo. (Ameer Alhalbi/AFP/Getty Images)

A U.S.- and Russian-backed cease-fire agreement that went into effect Monday was almost immediately violated, diluting hopes for an imminent halt to the relentless violence that has raged for the past five years and raising new questions about U.S. policies aimed at ending the war.

Residents and activists of the besieged rebel portion of Aleppo said that Syrian government helicopters had dropped barrel bombs on one neighborhood of the city and that loyalist forces were shelling a route intended to be used for the delivery of humanitarian aid. Pro-government media accused the rebels of launching a new attack in the southern province of Quneitra, and there were reports of airstrikes and artillery shelling in other parts of the country.

Secretary of State John F. Kerry hailed the U.S.-Russian accord on Syria that was announced in Geneva on Saturday as “a last chance . . . to save a united Syria.”

But as the reports of new violence trickled in, the most intractable and bloody of the Middle East’s wars seemed only to have become more complicated.

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)

The cease-fire follows a previous effort in February that seemed initially to herald a greater level of calm but gradually collapsed. Like that deal, the new agreement contains no penalties for violations, which enhances the advantage of the Syrian government and its Russian allies because they are the only ones on the battlefield capable of carrying out airstrikes against their opponents.

This accord also adds new complications in the form of a provision for the United States to carry out joint airstrikes with Russia against extremists whose positions are known to be entangled with the moderate rebels on some front lines.

That has resulted in confusion even on the part of Kerry as to what the agreement means. In comments widely disparaged by opposition supporters on social media, he seemed to indicate that Russia and the United States would jointly approve future airstrikes carried out by the Syrian government.

The State Department issued a statement retracting the remarks.

“To clarify: the arrangement announced last week makes no provision whatsoever for the U.S. and Russia to approve strikes by the Syrian regime, and this is not something we could ever envision doing,” the statement said.

In his comments, Kerry said the early violations did not mean the accord had failed. “It’s far too early to draw any definitive conclusions,” he said, adding that it may take “a day or two” to fully secure a cessation of hostilities.

The rebels did not reject the agreement outright but came out with a searing statement criticizing its lack of enforcement mechanisms, saying it would “leave room for the regime to take advantage of the situation to achieve military gains that they would have been incapable of achieving before.”

“Honestly we do not trust the regime and we do not trust the Russians,” said Maj. Jamil al-Saleh, who heads the U.S.-backed Tajamu al-Izza brigade in the Hama province town of Latamneh.

Halting the fighting is intended to be the first step in a series of measures including the rapid delivery of humanitarian aid and culminating in new negotiations for a political solution to the conflict. Whether the process will work depends to a large extent on whether Russia and the United States can bring pressure on their respective allies — the government and the rebels — and also whether Washington and Moscow are committed enough to make the deal stick.

Another glaring omission of the agreement is any mention of what will become of Syrian President Bashar al-Assad, indicating that Russia and the United States still do not see eye to eye on this key question.

Hours before the deal went into effect, Assad reiterated his determination to reconquer all of Syria from what he termed “terrorists,” signaling that he has no plans to stop fighting to crush the five-year-old rebellion against his regime.

“We as a nation . . . are delivering a message that the Syrian state is determined to recover all regions from the terrorists and restore security, infrastructure,” Assad said in remarks after attending prayers marking the Muslim Eid al-Adha holiday in Darayya, a Damascus suburb that was recently recaptured from the rebels after a four-year siege.

“We come today here to replace the fake freedom they tried to market at the beginning of the crisis . . . with real freedom,” he added, “not the freedom that begins with them and is sustained by dollars . . . and by some promises of positions.”

Yasir Ibrahim al-Yusuf, a member of the political office of the rebel Noureddine al-Zinki movement, said the armed opposition has raised many concerns about the details of the deal with the Obama administration, notably the absence of enforcement mechanisms or penalties for noncompliance by the Assad regime or the Russians.

A letter to the opposition delivered over the weekend by the U.S. special envoy for Syria, Michael Ratney, spelled out details similar to those outlined by Kerry and Russian Foreign Minister Sergei Lavrov in Geneva on Saturday. They include a cessation of hostilities, the delivery of humanitarian aid and the eventual launch of joint military operations by the United States and Russia against two agreed-upon terrorist groups — the former Jabhat al-Nusra and the Islamic State.

The letter offered no new enforcement measures other than the reporting mechanism established by the cease-fire agreement earlier this year, which collapsed within weeks amid escalating government airstrikes and eventual resumption of Russian bombing. The armed opposition is nonetheless committed to complying “because it is in­cred­ibly important that aid reaches people and that there is a decrease in the numbers of people dying,” Yusuf said. “Also, we are hoping this is the beginning of a political solution to the conflict.”

“The one assurance we have is that the Russians are very invested because they want to extricate themselves from this conflict as quickly as possible,” he added. “This is the one reason we are agreeing to the cease-fire. It seems everyone very much wants to make it work.”

According to the timetable laid out by Kerry and Lavrov, if the cease-fire holds for seven consecutive days and humanitarian aid flows unimpeded to besieged areas, then Moscow and Washington will start working out plans to conduct joint military operations.

Russia and the United States would then begin sharing intelligence and coordinating their attacks on Jabhat al-Nusra, the former al-Qaeda affiliate now known as Jabhat Fatah al-Sham, or Front for the Conquest of Syria. A number of opposition groups have been fighting side by side with the terrorist organization — either because it is the most powerful and successful force against Assad or because of some degree of ideological affinity in the case of some groups.

Kerry warned Monday that those who are party to the cease-fire will have to separate themselves. “Al-Nusra is al-Qaeda,” he said. “Al-Nusra is the sworn enemy of the United States . . .the Western world . . . others in the region. They have an external plotting agency plotting as we speak against some of our allies, friends and ourselves.”

Warning that opposition groups that stick with al-Nusra could be subject to U.S. or Russian airstrikes, Kerry said, “We cannot . . . somehow adopt the moral hazard of just because they fight fiercely, we’re going to somehow allow al-Qaeda to be the tip of our spear with regard to Assad.”

DeYoung reported from Washington. Zakaria Zakaria in Istanbul, Heba Habib in Stockholm and Carol Morello in Washington contributed to this report.