Top US and Ukrainian officials said Sunday they were making progress toward ending the Russia-Ukraine war but provided scant details as they discussed the American proposal to achieve peace despite concerns among many of Washington's European allies that the plan was far too conciliatory to Moscow.

US Secretary of State Marco Rubio said high-stakes talks in Geneva were “very worthwhile” and constituted the most productive day in “a very long time” on larger efforts to end the fighting.

“I feel very optimistic that we can get something done," Rubio said.

But he offered very little information on what was discussed. He also downplayed a Thursday deadline set by President Donald Trump for Ukraine to respond to the plan, saying simply that officials want to see fighting stop as soon as possible and that officials could keep negotiating Monday and beyond. He said that higher-level officials may eventually have to get involved.

“This is a very delicate moment,” Rubio said of what still needed to be worked out. “Some of it is semantics, or language. Others require higher-level decisions and consultations. Others, I think, just need more time to work through.”

The 28-point blueprint drawn up by the US to end the nearly four-year war has sparked alarm in Kyiv and European capitals. Ukrainian President Volodymyr Zelenskyy has said his country could face a stark choice between standing up for its sovereign rights and preserving the American support it needs.

The proposal acquiesces to many Russian demands that Zelenskyy has categorically rejected on dozens of occasions, including giving up large pieces of territory.

Concerns intensified after a bipartisan group of US senators said Rubio told them Saturday that the plan had originated with Russia and was actually a “wish list” for Moscow, rather than a serious push for peace.

The Ukrainian leader has vowed that his people “will always defend” their home.

German Chancellor Friedrich Merz said that he’d spoken to Trump and made clear there were some parts of the plans key European nations could agree on but others where they could not.

“I told him that we are fully in line with Ukraine, that the sovereignty of this country must not be jeopardized,” Merz said in an interview with DW.

Rubio called the US proposal a “living breathing document” that would continue changing. He also made, clear though, that any final product — once it’s ready — will still have to be presented to Moscow: “obviously, the Russians get a vote here.”

The head of the Ukrainian delegation, presidential chief of staff Andrii Yermak, said of the talks, “We have made very good progress and are moving forward to a just and lasting peace,” he said.

The rosy assessment of what was discussed defied Trump himself. Before talks began, the president blasted Ukraine for a lack of gratitude for US military assistance while shying away from criticizing Russia.

Trump has set a Thursday deadline for Ukraine to respond to the plan but also suggested it could slide if there was proof of real progress. He also said that the plan was not his final offer — without offering further details on what that meant.

“UKRAINE ‘LEADERSHIP’ HAS EXPRESSED ZERO GRATITUDE FOR OUR EFFORTS, AND EUROPE CONTINUES TO BUY OIL FROM RUSSIA,” Trump posted on his social media site Sunday morning.

After Trump’s post, Zelenskyy cheered U.S.-led efforts on security while also stressing that “the crux of the entire diplomatic situation is that it was Russia, and only Russia, that started this war.”

“Ukraine is grateful to the United States, to every American heart, and personally to President Trump,” Zelenskyy wrote in a post on Telegram, adding, “We thank everyone in Europe.”

“It is important not to forget the main goal – to stop Russia’s war and prevent it from ever igniting again,” he added.

Before convening with US officials, Yermak and his team also met with national security advisers from the UK, France and Germany. The allies have rallied around Kyiv in a push to revise the plan.

Alice Rufo, France’s minister delegate at the Defense Ministry, told broadcaster France Info before the talks began that key points of discussion would include the plan’s restrictions on the Ukrainian army, which she described as “a limitation on its sovereignty.”

“Ukraine must be able to defend itself,” she said. “Russia wants war and waged war many times in fact over the past years.”

On Sunday, Zelensky said that there was an understanding the US would take into account “a number of elements” in a peace deal that are important for Ukraine, but did not elaborate further.

“There have already been brief reports from the team about the results of the first meetings and conversations,” he said. “There is now an understanding that the American proposals may take into account a number of elements based on the Ukrainian vision and are critically important for Ukraine’s national interests.”

Polish Prime Minister Donald Tusk said Sunday that Warsaw was ready to work on the plan with the leaders of Europe, Canada and Japan, but also said that it “would be good to know for sure who is the author of the plan and where was it created."

Some U.S. lawmakers said Saturday that Rubio had described the plan as a Russian “wish list” rather than a Washington-led proposal. A US State Department spokesperson said that account was “blatantly false.” On his way to Geneva, Rubiothen took the extraordinary step of suggesting online that the senators were mistaken, even though they said he was their source for the information.

The issue was still causing a stir on Sunday.

The top Democrat on the US Senate Intelligence Committee, Sen. Mark Warner, told ABC’s “This Week” that the peace plan appeared to be “almost a series of Russian talking points,” had made Europeans “feel like they’ve been totally left high and dry” and had led to “ferocious pushback.”

Turkish President Recep Tayyip Erdogan announced that he would hold a phone call with Russian President Vladimir Putin on Monday. He said he would talk to Putin about reviving a previous deal from July 2022 that allowed Ukraine to safely ship exports of grain via the Black Sea.

The agreement stayed in place until the following year, when Putin refused to extend it, saying that a parallel agreement promising to remove obstacles to Russian exports of food and fertilizer hadn’t been honored.

“We had a grain corridor endeavor to open the path to peace,” Erdogan said, “Unfortunately we were only partially able to succeed. Tomorrow I will be asking Putin to revisit the endeavor.”