HANGZHOU, China — Efforts by the United States and Russia to forge a deal for a cease-fire in Syria and to coordinate their counterterrorism operations there faltered again Sunday, even as a major new Syrian-Russian offensive in the besieged city of Aleppo appeared to undermine key components of the proposed agreement.

After an anticipated news conference did not take place, Secretary of State John Kerry told reporters that his negotiations here with Russian Foreign Minister Sergei Lavrov snagged on “a couple of tough issues” – nearly identical to the language he used when the two failed to reach agreement in their last meeting, just over a week ago in Geneva. Officials said they would meet again Monday.

Kerry and Lavrov are in Hangzhou for a meeting of the Group of 20 economies, also attended by President Obama and Russian President Vladimir Putin. In his own news conference, Obama said he was skeptical that Russia and Syria would abide by any agreement but that a long-term solution to Syria’s civil war is still urgently needed.

“It is worth trying,” he said, because “there are women, children, innocent civilians who can get food and medical supplies to get some relief from the terror of constant bombings.” Citing “grave differences,” Obama said that “it’s premature for us to say there’s a clear path forward, but there’s the possibility, at least for us, to make some progress.”

The administration, which has long been reluctant to intervene directly in the civil war, nonetheless thinks that it is a distraction to what it considers the more important, separate battle against the Islamic State – in both Syria and Iraq – and that it must be resolved.

Kerry, with approval from Obama, traveled to Moscow in mid-July to propose an agreement under which the United States would share intelligence and coordinate its bombing of terrorist targets with Russia if Moscow would agree to ground the Syrian air force and stop its own bombing of U.S.-backed opposition forces.

Russia has complained of increasing overlap between the opposition and terrorist groups on the ground, and it said it was up to the United States to separate them before a deal could be struck. U.S. failure to do so has allowed the Russians to claim they are targeting only terrorists of the Front for the Conquest of Syria – or Jabhat Fatah al-Sham, the al-Qaida affiliate formerly known as Jabhat al-Nusra – and the Islamic State.

In comments to Russian journalists earlier in the day, Deputy Foreign Minister Sergei Ryabkov indicated that the separation of forces remains an issue. “There is a great phrase: Nothing is agreed until everything is agreed,” Ryabkov said. For months, he said, Russia has been asking the Americans for “a real, genuine demarcation between terrorists and the so-called opposition.”

But he insisted that the two sides were “close to agreement. … There is no basis to expect that all of this will collapse.”

On Aug. 26, Kerry met with Lavrov in Geneva to try to build momentum for the proposal. The two sides said then that they were “close” to agreement and that military and intelligence teams from both sides were ironing out “technical details.” Kerry predicted a deal could be reached within a week.

Since then, the bombardments have continued, including in the city of Aleppo – divided between rebels in the east and the government in the west – where the United Nations says nearly 2 million people are now without food, water and medical care. The northern route out of the city to Turkey, previously held by the rebels, has been cut by the government. U.N. envoy Staffan de Mistura has pleaded with all sides to allow aid to travel into the city along the corridor, but the rebels have refused unless government forces withdraw from what is known as Castello Road.

In a letter to rebel leaders Saturday, the U.S. liaison to the opposition groups, Michael Ratney, outlined the parameters of the Kerry proposal and asked for their agreement, Reuters reported.

In the meantime, though, the Syrian government over the weekend made important battlefield gains around Aleppo that challenge basic assumptions of a proposed cease-fire that was to freeze last week’s battle lines.

While Front-led forces last month broke through government encirclement from the south to join with opposition rebels occupying the eastern half of the city, that route now also appears to be closed as a conduit for aid, as the government launched a major push to retake it. Late on Sunday, Syrian forces, backed by Russian airstrikes, seized control of the area, known as the Ramouseh corridor, and fully laid siege to Aleppo.

Russian support for the offensive prompted U.S. officials to raise questions over whether the Russians could be relied upon to implement an agreement to freeze battle lines and ground the Syrian air force.

The Russians “must have known that if Ramouseh falls, the deal would have to be scrapped and started over,” said Faysal Itani of the Washington-based Atlantic Council. “The deal was not a good deal for the Russians. … If they have a chance to make gains on the ground and start over, it’s not bad for them.”

Beyond Aleppo, Russia and the Syrian government of Bashar al-Assad have continued to refuse to allow U.N. humanitarian aid convoys to enter more than a dozen besieged towns and cities held by various rebel forces. In recent days, two suburbs of the capital, Damascus, that have been under siege for years – Darayya and Moadamiya – have given in to government proposals to evacuate civilians and withdraw rebel forces.

De Mistura has criticized those arrangements, and outside experts have charged that the Russians and the Assad government have violated earlier agreements to allow aid to pass and have dragged their feet on a cease-fire until they can starve the besieged communities into submission.