In this photo released by the Syrian official news agency SANA, Syrian citizens and firefighters fight fires after rockets hit a hospital in Aleppo, Syria on Tuesday. (SANA/via AP)

U.S. and Russian military officials will sit in the same room 24 hours a day and jointly pore over maps and intelligence to monitor cease-fire violations in Syria under a new system they hope will save a fast-collapsing truce, Secretary of State John F. Kerry said Tuesday.

Under the new arrangement, which Kerry said could be finalized by Wednesday, lines will be drawn in and around Aleppo, scene of the heaviest recent fighting, to prevent new incursions or attacks from any party in the Syrian civil war.

The city will not be allowed to fall to the government, Kerry said. “If [Syrian President Bashar al-] Assad’s strategy is to somehow think he’s going to carve out Aleppo . . . I’ve got news for him,” he told reporters at the State Department.

He spoke just hours after another deadly strike on a hospital in Aleppo, this time by opposition forces. The official Syrian Arab News Agency said “scores” of people were killed in a rocket assault on government-held neighborhoods in the western part of the city, which has been divided between the opposing forces since 2012. Activists put the number of dead at 19.

Russian Foreign Minister Sergey Lavrov, right, welcomes Staffan de Mistura, UN Special Envoy of the Secretary-General for Syria, as they arrive for talks in Moscow on May 3. The UN’s envoy for Syria and Lavrov have began talks in Moscow about strengthening the faltering cease-fire in Syria. (Alexander Zemlianichenko/AP)

Kerry acknowledged rebel responsibility, saying that “today’s attack . . . appears to be rockets that have come from some area of opposition — we are trying to determine which.”

Over the past two weeks, government airstrikes and shelling, including last week’s bombing of a pediatric hospital in the city, have left at least 250 civilians dead.

Syria’s information minister, Omran al-Zoubi, warned that the government is ready to strike back against rebel shelling of civilian areas. Activists said opposition-held areas were also pummeled by Syrian government airstrikes Tuesday, and at least two people were killed in the city’s Fardous district.

“The bottom line is, there is no justification for this horrific violence,” Kerry said, “whether a member of the opposition retaliating or the regime in its brutality against civilians . . . We condemn any of these attacks, no matter who commits them.”

The U.N. Security Council on Tuesday passed a unanimous resolution calling for an end to attacks on health-care workers and health facilities worldwide.

“This resolution cannot end up like so many others, including those passed on Syria over the past five years: routinely violated with impunity,” Joanne Liu, international president of Doctors Without Borders, which supported the government-destroyed al-Quds hospital, told the council.

The renewed fighting, in Aleppo and elsewhere, has brought a cease-fire brokered by the United States and Russia in late February to the verge of collapse. After an urgent appeal by U.N. envoy Staffan de Mistura to do something to prevent that from happening, Kerry said he and Russian Foreign Minister Sergei Lavrov have come up with the new monitoring plan.

Washington and Moscow have ostensibly been monitoring the cease-fire since it began, but only from a distance. Russian military officers at their base in Hmeymim, Syria, and U.S. counterparts in Amman, Jordan, communicate by telephone and videoconference, and they have rarely met in person.

At a new coordination center in Geneva, “U.S. and Russian counterparts will be sitting at the same table,” Lavrov said at a Moscow news conference with de Mistura. “They will be looking at the same maps. They will be analyzing proposals and they will work together to make sure that any violations are nipped in the bud.”

The cease-fire and the lifting of sieges around civilian areas that have blocked delivery of humanitarian aid were supposed to pave the way for political negotiations between the opposition and the government to end the war and install a transition government in Syria. Two weeks ago, the opposition walked out of the talks to protest ongoing government violence.

The Obama administration, which halted all military cooperation with Russia after its incursion into Ukraine, has been reluctantly dragged back into coordination over Syria. The two governments back opposite sides in the long-running civil war, but the United States has tried to keep Moscow at arm’s-length.

“We have a responsibility — we, those who support the opposition . . . have an obligation to work with the opposition to keep the opposition from breaking” the truce, Kerry said. “Likewise, two parties — Iran and Russia — have a particular responsibility that they assumed at the same time that we assumed ours.”

Left unclear by both Kerry and Lavrov was what they proposed to do if and when they determine one side or the other has crossed the new lines they intend to delineate. Kerry, who met Tuesday at the White House with President Obama, has long favored a direct U.S. military response against Assad that Obama has resisted.

Kerry acknowledged that they were still grappling with what to do about Jabhat al-Nusra, the al-Qaeda affiliate whose forces are excluded from the cease-fire but are intermingled with some opposition forces around Aleppo.

“Are they somehow commingled? Are they fair game? These are the kinds of things that have to be worked out, so that there’s no misunderstanding” about “who is doing what, where, when and how,” Kerry said.

“We don’t control the terrorists,” he said. “They can obviously move and try to use other people as a human shield . . . that’s where it gets complicated.”

Cunningham reported from Istanbul.