WASHINGTON — A day after President Biden branded President Vladimir V. Putin of Russia a “war criminal” over civilian deaths in Ukraine, Secretary of State Antony J. Blinken on Thursday echoed his assessment and said Mr. Putin would be held accountable.

“Yesterday, President Biden said that, in his opinion, war crimes have been committed in Ukraine. Personally, I agree,” Mr. Blinken said, citing a list of horrific Russian attacks that have killed unarmed Ukrainians, including children. “Intentionally targeting civilians is a war crime.”

But the practical obstacles to punishing Mr. Putin are huge, experts said, though his battlefield commanders in Ukraine could be more vulnerable. Complicating matters is the fact that the United States does not officially recognize the International Criminal Court, which is the main forum for prosecuting war crimes.

Some experts said that declaring the Russian leader a war criminal could make it more difficult to negotiate a peace agreement with him, but that it might also give Ukraine and the West some leverage if Mr. Putin sought to bargain for immunity from any prosecution.

The back-to-back comments by Mr. Biden and Mr. Blinken marked a clear change in U.S. language on the subject after weeks of noncommittal statements by American officials even as Ukrainian hospitals and apartment blocks were pounded to rubble.

Two weeks ago, the White House press secretary, Jen Psaki, told reporters that the United States had “not made conclusions” about whether war crimes were being committed in Ukraine, saying the question was the subject of an official legal review.

Growing evidence of horrific Russian attacks on civilian targets — including the bombing on Wednesday of a Mariupol theater that may have sheltered hundreds of people driven from their homes — has made that position hard to sustain.

Legal experts said U.S. officials must be mindful of not seeming to prejudge complex legal issues that may come to trial, and Mr. Biden and Mr. Blinken both couched their assessments in personal terms, stopping short of statements of U.S. government policy.

“I think he is a war criminal,” Mr. Biden said in response to a reporter’s question on Wednesday.

A Senate resolution unanimously approved on Tuesday condemned Mr. Putin for “alleged war crimes” in Ukraine.

“The reason for all their caution is that for any crime, there’s an evidentiary standard that has to be met,” said Oona Hathaway, a professor of international law at Yale Law School who serves on a State Department legal advisory board. “If you’re having a trial, you can’t just say, Yeah we all pretty much assume that he knew what was going on.”

Ms. Hathaway said prosecutors would have to show that Russian commanders had intentionally targeted civilian structures, or struck them during attacks that failed to discriminate between civilian and military targets. In the case of Mr. Putin, prosecutors would have to demonstrate that he issued specific orders tied to those actions.

Apprehending and trying anyone proven guilty, not least the sitting president of a nuclear-armed nation, is another matter. “There’s no marshal service that goes with the International Criminal Court,” said Representative Tom Malinowski, Democrat of New Jersey and a former top State Department official for human rights.

But Mr. Malinowski and others said war crimes investigations can have a powerful deterrent effect. While Russian officials might hope that sanctions against them will someday be lifted, an indictment for war crimes brings a permanent stigma and risk of arrest.

With Russia’s military campaign bogged down and Ukraine claiming to have killed several Russian generals, Mr. Putin’s commanders in the field might have a reasonable fear of being captured and eventually tried for what amounts to mass murder. Frontline troops could also be demoralized by the official investigations.

“The hope is that it creates a disincentive for the most exposed people, who also happen to be the people closest to the fighting,” Ms. Hathaway said.

And it is possible that Mr. Putin would be deposed and could somehow fall into foreign hands. The former nationalist Serbian leader Slobodan Milosevic, accused of war crimes during the breakup of Yugoslavia, was arrested by Serbian authorities after his 2001 ouster from office and delivered to The Hague for prosecution. (He died during his trial in 2006.)

The concept of international justice for war crimes was established during the Nuremberg trials of Nazi German leaders after World War II. It is based today on the Geneva Conventions, a series of treaties governing the wartime treatment of civilians, prisoners of war and others, which have been adopted by every nation.

Although multiple bodies and nations are investigating possible war crimes in Ukraine, experts said the International Criminal Court offered the best chance for real accountability. Based in The Hague, the court was established in 1998 after separate tribunals prosecuted mass atrocities in Rwanda and the former Yugoslavia, demonstrating the need for a standing judicial body to handle such cases.

Last month, the top prosecutor of the International Criminal Court, Karim Khan, announced that he was opening an investigation into Russia’s invasion of Ukraine. Mr. Khan traveled this week to Poland and Ukraine to begin collecting evidence and met virtually with President Volodymyr Zelensky.

In an interview with CNN from Ukraine, Mr. Khan said he would investigate whether there were instances where Ukrainian forces mounted attacks from populated areas that could make them legitimate targets. “But even then, it’s no license to use cluster bombs or use disproportionate attacks in concentrated civilian areas,” he added.

The United States has had a fraught relationship with the court and is not among its 123 member nations. President George W. Bush revoked President Bill Clinton’s signature on its founding document, saying he would not accept the court’s jurisdiction over American troops abroad. President Barack Obama cooperated with the court but never sought to make the United States a member.

The administration of President Donald J. Trump was vividly hostile toward the body, which Secretary of State Mike Pompeo derided as a “kangaroo court” and biased against Israel. Mr. Trump even slapped sanctions on its top prosecutor and others after she began an inquiry into alleged war crimes by U.S. troops in Afghanistan.

“Traditionally, the U.S. has objected to assertion of jurisdiction by the I.C.C. over U.S. nationals because the U.S. never accepted the jurisdiction of the court,” said Todd Buchwald, the head of the State Department’s Office of Global Criminal Justice during the Obama administration. “The question is, how do we think about this now?”

Other bodies could prosecute alleged Russian war crimes. The United Nations or allied countries could establish special tribunals, and individual nations can also assert what is known as universal jurisdiction, a legal concept allowing a nation’s court to try people for human rights crimes. In January, a German court following the principle convicted a former security official for the Syrian government on torture charges.

But the Syrian, Anwar Raslan, had migrated to Germany, where he presumably did not expect to be identified and apprehended.

Russian officials are highly unlikely to make themselves vulnerable to such arrests.

“A very big problem is actually getting people in the dock,” said Matthew Waxman, a law professor at Columbia University who served in senior national security roles in the Bush administration.

“I’m very doubtful that Putin will ever find himself in The Hague,” he added.

Edward Wong contributed reporting.