WASHINGTON — Allied governments have been discussing how to secure the line of succession in Ukraine in the event President Volodymyr Zelensky is captured or killed by Russian forces, according to officials from multiple governments.

The concerns are primarily about making sure there is still an independent Ukrainian government in some form, even if Russia finds a way to install a puppet leadership in Kyiv, the capital. Having an independent leader to recognize, Western officials said, will help prevent any Russian-backed leaders from gaining legitimacy.

Mr. Zelensky’s presence and motivational speeches have been key factors in keeping up the morale of the Ukrainian military and people, and the officials said it was important that continued.

The focus on securing succession comes, in part, because Ukraine’s Constitution is unclear on the issue and because Mr. Zelensky has said he does not want to be evacuated. He memorably quipped “I need ammunition, not a ride.” Despite news reports, American officials deny they ever offered to evacuate the president or advised him to leave. And Western governments have applauded his resolve to stay and fight as Russian troops try to advance across the country.

The United States, Britain and the European Union would not recognize a government set up by Russia. Nevertheless, undermining a Moscow-controlled government in Kyiv will be easier for the United States and its allies if there is a legally recognized leader of a free Ukraine, rather than competing politicians vying for that role.

Some practical and legal issues are at play as well. The European Union and NATO countries have been largely making their military and economic donations public as a way to show support for Ukraine. European countries have sent automatic weapons, Stinger antiaircraft weapons, various anti-tank missiles and protective equipment to demonstrate that allies are intent on bolstering Ukraine’s ability to damage Russian military forces.

Continuing such public support is far easier with a functional government to accept the aid, even if it is operating in western Ukraine or as a government-in-exile in Poland or Romania.

The United States has a long history of covertly providing arms to insurgent groups around the world. Such a program for Ukraine — which would require a formal but secret finding from President Biden — remains a possibility. But the longer the organized military leads the fight against Russia, the more likely Ukraine will be able to keep control of all or part of the country.

Over the last week, intense discussions in the White House and in closed-door meetings on Capitol Hill have focused on how to provide aid to Ukraine should Russia take over the capital. In that situation, the administration currently plans to continue overtly supplying weaponry to the Ukrainians.

The strong public signal of support, opposed to secret weapons programs, should help strengthen Ukraine’s morale and demonstrate to Russia that the supply of weapons to the Ukrainian military was not going to stop, according to a person briefed on the discussion, who like others interviewed for this article spoke on the condition of anonymity to discuss sensitive conversations with the Ukrainian government.

American officials have pushed the Ukrainians to not allow senior officials in the line of succession to remain in the same place for long periods, and have also urged that they be moved to safer locations outside Kyiv, said a person briefed on the conversations.

U.S. and allied officials would like the Ukrainian government to set up a location for the leadership to use should Kyiv fall, according to multiple officials. A presidential retreat in the Carpathian Mountains in western Ukraine could possibly be used, but Ukrainian officials have not said if the facility is outfitted with bomb shelters and hardened communications capabilities.

Under the Ukrainian Constitution, the speaker, or chairperson, of parliament would succeed Mr. Zelensky as acting president. The current speaker, Ruslan Stefanchuk, is a pro-Western politician and former top aide to Mr. Zelensky. On Monday, Mr. Stefanchuk was pictured with Mr. Zelensky signing Ukraine’s application for European Union membership. And on Friday, he participated in a virtual meeting with the president of the European Parliament.

U.S. and European officials said Mr. Stefanchuk and others in the line of succession were expected to continue to oppose the Russian invasion.

Ukrainian officials have resisted suggestions from U.S. and European officials to relocate Mr. Stefanchuk but have said they understand the need to ensure a legal succession, according to two people briefed on the conversations.

On Thursday, Mr. Zelensky held a news conference in a room where sandbags were stacked against the windows for protection. While he did not talk about succession, he did raise the possibility of dying.

Ukrainian officials have publicly said they are not interested in discussing succession and are focused on fighting, and winning, the war. Maryan Zablotskyy, a member of parliament who is in Mr. Zelensky's party, said in an interview he had heard no discussions on the succession issue.

The approach before the invasion, when Ukrainian officials were publicly skeptical that Russia would attack, has quickly given way to a more cleareyed view of the situation. The Ukrainians are now ready to make wartime calls, say people briefed on the conversations.

For weeks ahead of the invasion, the United States and Britain warned about Moscow’s desire to push Mr. Zelensky from power. They discussed how succession in Ukraine would work if they needed to counter a Russia-backed coup.

And despite the public rhetoric ahead of the invasion that the United States was overstating the threat, Mr. Zelensky privately took the warning more seriously and realized the Russians were intent on capturing or killing him, according to American officials.

During a visit to Kyiv in January, William J. Burns, the C.I.A. director, discussed the intelligence about the threat to Ukraine with Mr. Zelensky, according to a person briefed on the meeting. When Mr. Zelensky raised the issue of his family’s safety, Mr. Burns replied that he needed to take both the threats to Ukraine and to himself seriously.

Since the invasion began in earnest, Russian officials have made plain that their intention is to push out the current government in Ukraine and install one friendly to Moscow. The State Department accused Russia of developing lists of Ukrainian politicians to arrest and seize as their forces moved forward.

“The importance of Zelensky’s personality under current circumstances is beyond doubt,” said Khrystyna Holynska, a Ukrainian who co-wrote a recent essay in The Hill newspaper about the succession issues. “If something happens to him, it would be very important to send a crystal clear message about who is leading the country now, how the government will be run.”

Ms. Holynska, a researcher at the RAND Corporation, said while it might not be wise for Ukraine to publicize plans to relocate the government, she hoped it was ready to operate in locations outside Kyiv.

Beyond Mr. Stefanchuk, the speaker of Ukraine’s parliament, the line of succession is not entirely clear, said Ms. Holynska. When Mr. Zelensky and Mr. Stefanchuk became sick with Covid in 2020, Ukrainian legal scholars said the prime minister, Denys Shmyhal, would be third in line to take over.

The Ukrainian Constitution creates the positions of first deputy and deputy chairperson to take over the duties of the parliament speaker, though it does not explicitly say they are in a line of presidential succession.

“People should know who’s next in line,” Ms. Holynska said. “Right now is very Zelensky focused. He’s in the news, he’s everywhere. Losing this image of a leader will be not good for the resistance, for the will to fight, for the spirit in Ukraine.”

Andrew E. Kramer in Kyiv and Safak Timur in Istanbul contributed reporting.