GENEVA (AP) — The United States and Russia early Saturday announced a breakthrough agreement on Syria that foresees a nationwide cease-fire starting on Monday, followed a week later by an unlikely new military partnership targeting the Islamic State and al-Qaida as well as the establishment of new limits on President Bashar Assad's forces.

After a daylong final negotiating session in Geneva, U.S. Secretary of State John Kerry said shortly after midnight Saturday that the plan could reduce violence in Syria and lead to a long-sought political transition, ending more than five years of bloodshed. He called the deal a potential "turning point" in a conflict that has killed as many as 500,000 people, if complied with by Syria's Russian-backed government and U.S.-supported rebel groups.

The cease-fire begins at sundown Sept. 12, Kerry said, coinciding with the Muslim Eid al-Adha holiday.

"Today the United States and Russia are announcing a plan which we hope will reduce violence, ease suffering and resume movement toward a negotiated peace and a political transition in Syria," Kerry said. "We are announcing an arrangement that we think has the capability of sticking, but it is dependent on people's choices."

"It has the ability to stick, provided the regime and the opposition both meet their obligations, which we — and we expect other supporting countries — will strongly encourage them to do," he added.

Kerry's negotiating partner, Russian Foreign Minister Sergey Lavrov, confirmed the agreement and said it could help expand the counterterrorism fight and aid deliveries to Syrian civilians under U.N. auspices that have been stalled for weeks. He said Syrian President Bashar Assad's government was informed of the accord, and prepared to comply.

"The United States is going the extra mile here because we believe that Russia, and my colleague, have the capability to press the Assad regime to stop this conflict and to come to the table and make peace," Kerry said, citing a number of recent meetings with Lavrov.

"This is just the beginning of our new relations," Lavrov said.

The deal culminates months of frenetic diplomacy that included four meetings between Kerry and Lavrov since Aug. 26, and a lengthy face-to-face in China between Presidents Barack Obama and Vladimir Putin. The arrangement hinges on Moscow pressuring Assad's government to halt all offensive operations against Syria's armed opposition in specific areas, which were not detailed. Washington must persuade "moderate" rebels to break ranks with the Nusra Front, al-Qaida's Syria affiliate, and other extremist groups.

The military deal would go into effect after both sides abide by the truce for a week and allow unimpeded humanitarian deliveries. Then, the U.S. and Russia would begin intelligence sharing and targeting coordination, while Assad's air and ground forces would no longer be permitted to target Nusra any longer; they would be restricted to operations against the Islamic State.

The arrangement would ultimately aim to step up and concentrate the firepower of two of the world's most powerful militaries against Islamic State and Nusra, listed by the United Nations as terrorist groups.

Both sides have failed to deliver their ends of the bargain over several previous truces.

But the new arrangement goes further by promising a new U.S.-Russian counterterrorism alliance, only a year after Obama chastised Putin for a military intervention that U.S. officials said was mainly designed to keep Assad in power and target more moderate anti-Assad forces.

Russia, in response, has chafed at America's financial and military assistance to groups that have intermingled with the Nusra Front on the battlefield. Kerry said it would be "wise" for opposition forces to separate completely from Nusra, a statement Lavrov hailed.

"Going after Nusra is not a concession to anybody," Kerry said. "It is profoundly in the interests of the United States."

The proposed level of U.S.-Russian interaction has upset several leading national security officials in Washington, including Defense Secretary Ash Carter and National Intelligence Director James Clapper, and Kerry only appeared at the news conference after several hours of internal U.S. discussions.

After the Geneva announcement, Pentagon secretary Peter Cook offered a guarded endorsement of the arrangement and cautioned, "We will be watching closely the implementation of this understanding in the days ahead."

At one point, Lavrov said he was considering "calling it a day" on talks, expressing frustration with what he described as an hours-long wait for a U.S. response. He then presented journalists with several boxes of pizza, saying, "This is from the U.S. delegation," and two bottles of vodka, adding, "This is from the Russian delegation."

The Geneva negotiating session, which lasted more than 13 hours, underscored the complexity of a conflict that includes myriad militant groups, shifting alliances and the rival interests of the U.S. and Russia, Saudi Arabia and Iran, and Turkey and the Kurds.

Getting Assad's government and rebel groups to comply with the deal may now be more difficult as fighting rages around Aleppo, Syria's most populous city and the new focus of a war that has killed as many as 500,000 people.

Assad's government appeared to tighten its siege of the former Syrian commercial hub in the last several days, seizing several key transit points. Forty days of fighting in Aleppo has killed nearly 700 civilians, including 160 children, according to a Syrian human rights group.

Kerry outlined several steps the government and rebels would have to take. They must now pull back from demilitarized zones, and allow civilian traffic and humanitarian deliveries — notably into Aleppo.

"If Aleppo is at peace, we believe that the prospects for a diplomatic solution will brighten," he said. "If Aleppo continues to be torn apart, the prospects for Syria and its people are grim."

But as with previous blueprints for peace, Saturday's plan appears to lack enforcement mechanisms. Russia could, in theory, threaten to act against rebel groups that break the deal. But if Assad bombs his opponents, the U.S. is unlikely to take any action against him given Obama's longstanding opposition to entering the civil war.