Diplomats from the U.S. and Ukraine thrashing out changes to a Trump-backed peace plan have left thorny decisions over territory to their countries’ presidents, according to European officials familiar with the discussions.

Officials from Washington left the Geneva meeting over the weekend expressing optimism about the modifications during what it called “constructive talks.”

But significant sticking points remain , in particular what happens to Ukrainian territory coveted by the Kremlin in eastern Ukraine that Russia hasn’t been able to capture in nearly four years of attritional warfare.

Kyiv says surrendering the rest of the Donbas that it still holds, as proposed by the plan, including heavily fortified strategic cities, would leave Ukraine vulnerable to future assaults by Moscow.

Negotiators found common ground on some matters at issue in the U.S. plan to end the war, and narrowed the proposed deal to 19 points from 28, according to European diplomats.

But some of those officials, who were briefed on the talks, said that questions over territory would now be dealt with at the presidential level.

Ukrainian President Volodymyr Zelensky said Monday his country had managed “to keep extremely sensitive points on the table” during discussions in Geneva on Sunday.

The main problem was the Kremlin’s demand for international legal recognition of the territory it has taken from Ukraine, he said.

“It is crucial to support the principles on which Europe stands, that borders cannot be changed by force,” Zelensky told Swedish lawmakers via video link.

President Trump on Monday expressed cautious optimism about a breakthrough in the talks in a post on his Truth Social platform. “Is it really possible that big progress is being made in Peace Talks between Russia and Ukraine???,” he wrote. “Don’t believe it until you see it, but something good may just be happening.”

Both the U.S. and Ukraine said that both the countries’ presidents would need to sign off on the final framework.

Public statements by the U.S. and Ukrainian delegations on the outcome of the meeting differed.

While the White House said that the revised plan “meaningfully addresses Ukraine’s core strategic requirements,” the Ukrainian summary of the meeting was less definitive. “Both sides reiterated their readiness to continue working together to secure a peace that ensures Ukraine’s security, stability, and reconstruction,” it said.

Further complicating matters: the closer the U.S. plan looks to allay Kyiv’s concerns, the less likely it is that Russia will accept it. Russian President Vladimir Putin on Monday said that the American proposals on Ukraine, in their original form, could form the basis of a settlement.

Here are the main sticking points to Ukraine accepting the plan.

Vague security guarantees

The U.S. plan demands that Kyiv surrender heavily defended territory in its eastern Donbas region, including strategically important cities that Moscow.

On Monday, Zelensky said the main problem with the multipoint plan was the Kremlin’s demand for international legal recognition of the territory it has taken from Ukraine.

Ukraine says that giving up the 11% of the Donbas it still holds, a longstanding Kremlin cease-fire condition, would make it easier for Russia to launch a future attack on the country.

Under the U.S. plan, America would recognize the surrendered territory as Russian but the area would be a demilitarized zone from which Russian forces would be barred.

As for the rest of the Donbas, which is currently under Russian occupation, the U.S. would recognize it as “de facto” Russian, the plan says.

Top European officials said over the weekend that negotiations between Ukraine and Russia should start with a cease-fire along the current front lines.

Vague security guarantees

The U.S. plan hinges on an American security guarantee which is meant to answer the key concern of Ukraine and Europe: how to ensure that Russia doesn’t resume the war in future and seek to conquer new parts of Ukraine.

Should Moscow attack Ukraine in future, the U.S. and several European countries would be involved in a “decisive coordinated military response,” the plan says.

But the extent and strength of that response isn’t spelled out, nor is the nature of the U.S.’s role.

The White House said Sunday that the revised framework included “strengthened security guarantee architecture” but didn’t say what that amounted to.

Europe and the U.S. have avoided direct involvement in the conflict so far. Ukraine fears that in the event of a fresh war, its Western partners wouldn’t offer sufficient protection to deter or overcome Russian aggression.

A renewed Russian invasion of Ukraine would trigger the snapback of international sanctions on Russia and recognition of any new Russian territorial claims would be revoked, the plan stipulates.

NATO ambitions

The U.S. plan would block Ukraine from membership of the North Atlantic Treaty Organization and its mutual defense pact. It also creates an expectation that the military alliance wouldn’t expand. No NATO troops could be stationed in Ukraine.

Kyiv’s chances of joining NATO remain low, though its constitution still lists it as a goal.

After Sunday’s talks, Rubio said that issues directly involving NATO and the European Union had been set aside for now. However, the Europeans are pushing for wording in the plan that would allow them to send a reassurance force to Ukraine after a peace deal, something Moscow has opposed. Such a force would never be under NATO command or allowed into Ukraine during peacetime, the Europeans say.

“The negotiations were a step forward, but there are still major issues which remain to be resolved,” Finland’s president, Alexander Stubb , wrote on X on Monday after speaking with Zelensky. “Any decision falling in the remit of EU or NATO will be discussed and decided by EU and NATO members in a separate track.”

The size of Ukraine’s military

The U.S. plan places a 600,000-person cap on the size of the peacetime Ukrainian military, while a European plan proposes a limit of 800,000.

This provision is less of an obstacle for Ukraine’s ability to defend itself; the country had a much smaller standing army prewar. Ukraine doesn’t publish figures on the strength of its armed forces but Zelensky put it at 880,000 personnel in January.

Maintaining a large peacetime army would strain Ukraine’s budget. But any deal would have to include robust Western guarantees for Kyiv to accept such terms.

The U.S. plan places no cap on armed forces in Russia, which launched its full-scale invasion of Ukraine in 2022.

What to do with frozen Russian assets

The U.S. plan wants to tap the roughly $300 billion in Russian sovereign assets that were immobilized in early 2022, in part for a U.S.-led reconstruction plan in Ukraine and in part for U.S.-Russian joint investment projects.

However, most of the funds are frozen in the EU under sanctions, limiting U.S. control. The Europeans want all the money to be used for Ukraine and are working on a loan, backed by the assets, for Kyiv. European officials said this weekend that decisions on the assets can only be taken with the direct involvement of the EU.

Zelensky urged Europe on Monday to use the Russian assets to make the aggressor pay for the war. “Keep pressure on Russia, Russia is still killing people,” he said.

Write to Laurence Norman at laurence.norman@wsj.com