Amid the setbacks for Ukraine from President Trump’s meeting in Alaska with President Vladimir V. Putin of Russia, officials in Kyiv found one glimmer of hope. They seized on a U.S. proposal to include security guarantees for Ukraine, designed to deter future Russian aggression, in a potential peace deal.

Mr. Trump conveyed the proposal to President Volodymyr Zelensky of Ukraine in a call early Saturday after the meeting. It would enlist Kyiv’s Western partners to guarantee Ukraine’s defense against new Russian attacks, according to European leaders who participated in the call.

“The good news is that America is ready to participate in such security guarantees. It is not leaving it to the Europeans alone,” Chancellor Friedrich Merz of Germany said after the call. That marks a shift from Mr. Trump’s earlier stance of avoiding any U.S. involvement in Ukraine’s postwar security.

Mr. Merz and other European leaders were set to meet virtually on Sunday afternoon to discuss the aftermath of the Alaska summit, including potential security guarantees. In a show of support for Ukraine, the chancellor and other European officials announced that they would join Mr. Zelensky when he meets with Mr. Trump on Monday in Washington.

While the specifics of the U.S. proposal remain unclear, Mr. Trump said Mr. Putin agreed that Ukraine should have strong security guarantees after a settlement, though not under NATO, two senior European officials who were briefed on the call have said. American troops might participate, Mr. Trump told the Europeans.

Should Mr. Trump’s proposal come to fruition, it would mark a win for Ukraine, which has long sought postwar security guarantees to prevent a future Russian invasion, but has so far received little beyond vague commitments.

“We consider the search for mechanisms of security guarantees for Ukraine a positive moment at this stage,” Yehor Chernev, the deputy chairman of the Ukrainian Parliament’s defense and intelligence committee, said in a text message.

But Mr. Chernev and other Ukrainian lawmakers cautioned that they remained confused about what exactly Mr. Trump had in mind and what Mr. Putin may have agreed to. The officials expressed worries that Mr. Trump may have misread what Moscow would be willing to accept and overstated his own proposal to Ukraine. White House officials did not immediately reply to a question about those concerns.

An official briefed on Mr. Trump’s call to Mr. Zelensky said the Ukrainian leader would aim to seek clarity on potential security guarantees when he visits Washington on Monday. He will also seek answers on Mr. Trump’s unexpected shift away from pursuing a cease-fire to instead call for a sweeping peace deal that would likely see Ukraine cede unconquered territory to Russia.

Besides Mr. Merz, others who announced they will also travel to Washington for Monday’s meeting included President Emmanuel Macron of France; Prime Minister Keir Starmer of Britain; Prime Minister Giorgia Meloni of Italy; President Alexander Stubb of Finland; NATO Secretary General Mark Rutte; and Ursula Von der Leyen, the head of the European Commission.

Ms. Meloni said that Mr. Trump, in his call with Mr. Zelensky and European leaders, had drawn on her earlier idea of guarantees modeled on Article 5 of the NATO pact which stipulates that an attack on one ally would be defended as an attack on all.

While Ukraine would not join NATO under such guarantees, its Western allies would abide by “a collective security clause that would allow Ukraine to benefit from the support of all its partners, including the United States, ready to take action if it is attacked again,” Ms. Meloni said in a statement following the call.

The idea, while appealing to the Ukrainians, has left them questioning its viability.

Given Russia’s strong opposition to Ukraine joining NATO and receiving the defense guarantees enshrined in Article 5, why would it agree to see Kyiv benefit from those guarantees in all but name? And if Moscow agreed to strong security guarantees outside of NATO, as suggested by Mr. Trump, would that imply it considers those guarantees ineffective without the alliance’s backing and therefore not a real deterrent?

Oleksandr Merezhko, chairman of the foreign affairs committee in the Ukrainian Parliament, said Ms. Meloni’s idea was “too vague” and left room for multiple interpretations which did not necessarily guarantee that Ukraine’s allies would immediately come to its defense if Russia were to ever invade again.

Solomiia Bobrovska, a member of the Ukrainian Parliament’s defense and intelligence committee, said that Ms. Meloni’s idea could only be interpreted as a commitment to provide more financial aid to Ukraine or to send additional ammunition.

Ukraine does not want to sign another Budapest Memorandum, a pledge signed in 1994 that was meant to protect the country after it gained independence — but clearly failed.

Under that accord, Ukraine agreed to give Russia back old Soviet nuclear weapons in exchange for security guarantees from Russia, the United States and Britain. But the agreement did not detail those guarantees, and included no promise of military assistance in the event of an attack. Ukrainian officials say the lack of specificity gave Russia free rein to attack their country, as it did starting in 2014.

“In order to avoid the fate of the Budapest Memorandum, these guarantees must be legally binding and also provide for specific steps and an algorithm of actions by the guarantors in the event of repeated aggression against Ukraine,” Mr. Chernev, the lawmaker, said.

One concrete guarantee that Ukraine has been seeking is the presence of Western troops on the ground — an idea that Mr. Trump appeared to entertain, despite previously opposing.

Ukraine’s European allies have already made headway in that direction. In the spring, a group of countries including France, Britain and Germany formed a “coalition of the willing” to help safeguard an eventual peace. Some of the countries have said that to do so, they would be willing to send troops to Ukrainian soil after the conflict ends. But the contours of that force have not been ironed out, and details of who is willing to do what remain scarce.

The coalition is set to meet on Sunday afternoon, with participants expected to discuss security guarantees for Ukraine, including the possibility of deploying troops.

Steven Erlanger contributed reporting from Berlin.