Secretary of State John Kerry has said the United States and Russia have agreed to “concrete steps” for trying to revive a crumbling ceasefire in Syria, following a day of marathon talks in Moscow.

Neither side would share details, but Kerry’s announcement appeared to suggest that Russia and the U.S. reached a preliminary deal to coordinate airstrikes together against jihadist groups in Syria, potentially representing a major shift in U.S. policy in the conflict.

Kerry made the announcement alongside his Russian counterpart, Sergey Lavrov, at a news conference in Moscow after the lengthy discussions that stretched throughout Thursday.

Before the talks, the Washington Post leaked a proposal document allegedly outlining what Kerry would offer to Russia. State Department officials have indicated the document was authentic, while declining to comment directly on it.

At Thursday's news conference, Kerry would not confirm whether the steps agreed with Moscow were the same as those laid out in the leaked proposal, though he strongly implied that they dealt with the same issues targeted in the proposal.

The proposal published in full on the Post’s website calls for the U.S. and Russia to collaborate militarily together against Jabhat al-Nusra, Al-Qaeda’s affiliate in Syria -- something long desired by Moscow. In return, the proposal would require that the Syrian air force be largely grounded.

At the news conference, Kerry first described the two main causes of the breakdown of Syria’s ceasefire as indiscriminate bombing by the Syrian government and offensives by al-Nusra. Kerry then said that he and Lavrov had “agreed to steps that if implemented in good faith can address the serious problems that I just described about the cessation.”

Kerry said that Russia and the U.S. would not be making thee steps public, saying that is “because we want them to work.” He added that the steps would have to be further developed in the coming days.

Kerry's statements suggested that the proposal leaked to the Post had been accepted in some format at least in principle by the Russians. In the leaked document, Russia and the U.S. would create a “Joint Implementation Group” based in Amman, Jordan, that would coordinate attacks on al-Nusra by Russian and U.S. military forces. Assad’s air force meanwhile would be forbidden from flying sorties in areas where al-Nusra is concentrated or in opposition-controlled areas where al-Nusra is operating.

If carried out, the proposal would represent major shifts for both Russia and the U.S. .

The U.S has regularly accused Russia and the Assad regime of using claims that it is targeting al-Nusra and the Islamic State as cover for actually hitting moderate opposition groups. The deal, if implemented, appeared to seek to end this possibility.

The removal of the Syrian air force from much of the battlefield would be a major breakthrough; continued bombing of groups involved in the ceasefire, as well as of civilians, has previously led opposition groups to suspend their participation in U.N.-backed peace talks. At the same time, U.S. officials have said al-Nusra was undermining the ceasefire by attacking the regime, often dragging more moderate groups into offensives and thereby breaching the ceasefire of which it was not a part and creating what Kerry called a "cycle of excuses" for more violence.

The potential deal was therefore meant to target these two problems.

The proposal has however been criticized as risking allying the U.S. with Syrian president Bashar al-Assad by helping Russia strike a group that has been one of the Syrian government's most formidable opponents and that is entwined with many other rebel groups. Kerry has also been criticized for putting too much faith in the Russians to uphold their side of the deal.

On Thursday, Kerry made it clear that in theoir talks the two sides had not been able to yet persuade one another the deal could or would definitely be implemented, but he suggested the groundwork was sufficient to try.

Kerry said he wanted to emphasize that the deal was “not based on trust” and that there were specific steps that would have to be shown to have been carried out. But he said that the deal if carried out in good faith “has the opportunity to change the playing field significantly.”

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