Behind the jubilation over the latest release of Americans unjustly imprisoned abroad are growing concerns that a new era of hostage diplomacy has emerged, with U.S. adversaries concluding that it pays to arrest innocent Americans and trade them for convicted smugglers, hackers, spies — and even killers.

While loath to criticize any specific swap, current and former U.S. officials worry that strongmen like President Vladimir V. Putin of Russia are exploiting America’s willingness to horse-trade for its citizens. Among those returned to Moscow on Thursday in a deal involving 24 prisoners and seven nations was a Russian assassin serving a life sentence in Germany for gunning down a Kremlin enemy in a Berlin park.

“I worry about the incentive this gives nations to abduct Americans,” said Adam Hickey, a former deputy assistant attorney general in the national security division of the U.S. Justice Department. Speaking of the trend toward hostage trades generally, he added: “I don’t see how this doesn’t lead to an escalation or increase” in the practice by foreign governments.

But practical alternatives are hard to come by, Mr. Hickey and other experts concede.

In response to Thursday’s deal, former President Donald J. Trump suggested that President Biden had set a “bad precedent” by paying too high a price to Mr. Putin.

Yet Mr. Trump offered no evidence that he could have struck a better deal, apart from a false claim that he had given up “NOTHING” as president to win the release of Americans imprisoned abroad. In fact, Mr. Trump traded prisoners with U.S. adversaries, including Iran and the Afghan Taliban, on at least four occasions.

In a joint statement, Speaker Mike Johnson and the Senate minority leader, Mitch McConnell, called for “serious action to deter further hostage-taking by Russia, Iran, and other states hostile to the United States.” The two Republicans offered no specifics about what steps should be taken, however.

Some national security experts even muse about whether the U.S. government should declare that it will not bargain for the release of anyone who disregards warnings against visiting Russia, Iran, North Korea and other high-risk countries.

One Biden administration official who has participated in prisoner-swap negotiations, and who spoke on the condition of anonymity to discuss sensitive matters, said it was hard to imagine an American president adopting such a position.

To the contrary, Mr. Biden has now presided over several such exchanges, vowing that he will never abandon helpless Americans. In late 2022, he approved a deal with Russia that swapped the notorious Russian arms dealer Viktor Bout for the basketball star Brittney Griner.

Mr. Biden also struck a deal with the Kremlin in April 2022, sending home an imprisoned Russian drug trafficker in return for a U.S. Marine arrested in Moscow. And he traded multiple prisoners with Iran last fall.

In every case, U.S. officials said the released Americans had been imprisoned on fabricated charges or given sentences wildly disproportionate to any actual crimes, as in the case of Ms. Griner, who was sentenced to 9 years in prison for possessing trace amounts of cannabis oil.

Officials and experts agree that these recent cases reflect what Secretary of State Antony J. Blinken in February called a “rising trend” in which American enemies are “wrongfully detaining people, often as political pawns.”

Mr. Blinken warned Mr. Biden of the problem in the opening weeks of his presidency, according to the Biden administration official. In July 2022, Mr. Biden declared hostage-taking and “wrongful detention” to be a national emergency.

The State Department considers wrongful detention to include groundless arrests, unfair trials and egregious sentencing. That designation sends cases to the U.S. envoy for hostage affairs, Roger D. Carstens.

Terrorist groups and foreign governments have snatched Americans with ransoms in mind at least since the 1793 Barbary Pirates hostage crisis, when the U.S. paid for the return of more than 100 citizens held in North Africa.

In 1979, Iranian revolutionaries imprisoned more than 50 U.S. Embassy workers, demanding the release of frozen Iranian assets. In the 1980s, Lebanese militants kidnapped dozens of Americans in Beirut to deter U.S. military action against Hezbollah. And in the past two decades, Al Qaeda and the Islamic State kidnapped numerous Americans, often executing them after the U.S. government refused to negotiate.

But in roughly the past 10 years, as foreign terrorist threats have receded, the imprisonment of Americans by hostile governments on false or inflated legal charges has risen sharply, according to Danielle Gilbert, an assistant professor at Northwestern University who studies so-called hostage diplomacy.

Ms. Gilbert said that the practice appeared to gain momentum under President Barack Obama, who approved a prisoner swap with Tehran as part of the 2015 Iran nuclear deal, and dramatically accelerated under Mr. Trump.

Beginning with Mr. Trump and continuing into Mr. Biden’s term, such cases have increasingly become public dramas. Families of prisoners, now often counseled by public-relations or diplomatic professionals such as Bill Richardson, until his death last year, increasingly reject traditional advice to lie low while government officials quietly do their work. Instead, they now enlist celebrities and the media to help pressure U.S. officials to “do whatever it takes,” as they often say. In a public statement after the release this week of the former security consultant Paul Whelan, jailed in Russia since 2018, his family noted that “it was media attention that helped to finally create critical mass and awareness within the U.S. government.”

Some worry the attention could benefit the bad guys. In their statement on Thursday, Mr. Johnson and Mr. McConnell warned that, absent strong new deterrent action, “the costs of hostage diplomacy will continue to rise” and that “trading hardened Russian criminals for innocent Americans does little to discourage Putin’s reprehensible behavior.”

Thursday’s deal took many months to orchestrate in part because senior German officials had similar concerns about freeing Vadim Krasikov, the convicted assassin whose release Mr. Putin had long demanded.

Christian Mihr, an official with Amnesty International in Germany, said the deal had left a “bitter taste” because it sent a convicted murderer home to Russia in exchange for political dissidents and journalists. The Russian government, Mr. Mihr warned in a statement reported by Agence France-Presse, “could feel emboldened to carry out further political arrests and human rights violations with no fear of the consequences.”

U.S. officials say they share such concerns. Briefing reporters on Thursday, Jake Sullivan, the national security adviser, called it “a question that we grapple with every time that we look at the hard decisions involved in one of these exchanges. It is difficult to send back a convicted criminal to secure the release of an innocent American.”

But Mr. Sullivan said the evidence was not “clear cut” that “a lot more people get taken because we do these exchanges.”