Lawmakers are appealing to President Joe Biden to use the remaining time in his term to work for the release of Americans who are “wrongfully detained” in China, saying U.S. officials need to do more to help secure their freedom.

“There are more Americans wrongfully detained in China than in any other country. American citizens are serving long prison terms in China; most were sentenced with acute due process irregularities or on spurious charges,” Republican Rep. Chris Smith of New Jersey and Democratic Sen. Jeff Merkley of Oregon wrote in a letter to the president.

“Many are severely mistreated in detention and have developed serious physical and mental health problems due to a lack of adequate nutrition or medical care in Chinese prisons. They deserve tenacious advocacy to gain their release,” said Smith and Merkley, co-chairs of the Congressional-Executive Commission on China.

The lawmakers recently oversaw hearings in which families of those detained spoke about how their loved ones are suffering psychologically and physically without legal recourse.

China’s embassy in Washington did not immediately respond to a request for comment. China has denied the allegations and says it has treated detained Americans properly and respected their legal rights.

Human rights groups have made similar allegations about the status of Americans held behind bars in China. The precise number of U.S. citizens who are detained under questionable circumstances remains unclear, and estimates vary. Families of imprisoned Americans and rights advocates say the U.S. government should expand its list of those deemed to be “wrongfully detained.”

The State Department recognizes three Americans as unjustly detained in China, including David Lin, a pastor who was released last month. The other two Americans are Kai Li and Mark Swidan.

Dui Hua, a rights group that advocates for detainees in China, estimates there are more than 200 Americans “under coercive measures” in China.

Li, from Long Island, New York, was detained in 2016 and sentenced to 10 years in 2018 for espionage. His family says the charges are baseless. Swidan, a Texas businessman, was detained in 2012 and convicted on drug-related charges in 2019. His advocates say he was not in China at the time of the alleged crime.

In their letter to Biden, the lawmakers said Li has spent more time behind bars than three Americans recently released from prison in Russia, Paul Whelan, Evan Gershkovich and Alsu Kurmasheva, combined. “This must change,” they wrote.

“The release of Americans should be a top priority item in any upcoming meetings you have with the PRC [People’s Republic of China] General Secretary and President Xi Jinping,” the letter stated.

The Chinese government uses “exit bans” to unjustly detain Americans “either to settle economic disputes or to coerce their relatives to return to China to face alleged crimes,” the lawmakers wrote.

“If the Chinese government wants to improve relations with the United States, it should release Americans who are wrongfully imprisoned without condition and unilaterally end the use of ‘exit bans,’ a form of de facto hostage-taking,” they said.

The lawmakers’ letter also asks Biden to meet with the families of Americans detained in China, and to have White House and State Department officials meet with them more frequently to share information about their cases and be more transparent about potential diplomatic avenues to win their loved ones’ release.

The lawmakers called for U.S. consular officials in China to visit the detained Americans more often, “secure longer and more frequent visits with their families, and ensure proper access to legal representation and health care.”

At a hearing last month before the commission, Li’s son Harrison Li said his father suffered a stroke and lost a tooth while in prison and that he was locked in a cell for most of three years when China imposed drastic restrictions during the Covid pandemic.

He told the lawmakers that precious time will be lost after a new administration takes office in January and that now was the time to push for his father’s release.

“The next few months will be critical before President Biden leaves office to … negotiate a fair deal that will finally bring my dad home,” he said.