US President Joe Biden called on the Taliban Sunday to “immediately release” the man believed to be its last American hostage, withholding any hope of recognizing the militants’ rule in Afghanistan until he is free. The announcement came on the eve of the two-year anniversary of the kidnapping of Mark Frerichs, a US Navy veteran who had spent a decade in Afghanistan working as a civil engineer.

“The Taliban must immediately release Mark before it can expect any consideration of its aspirations for legitimacy. This is not negotiable,” Biden said in a statement.

“Threatening the safety of Americans or any innocent civilians is always unacceptable, and hostage-taking is an act of particular cruelty and cowardice,” he added.

The United States completed its withdrawal from Afghanistan in August, closing more than two decades of war that began with the US-led invasion following the September 11, 2001 attacks and ended with the Taliban back in power. Washington has repeatedly told the Taliban it will have to “earn” legitimacy before being recognized by the international community.

According to reports, Taliban negotiators have previously suggested a prisoner exchange, swapping Frerichs for Bashir Noorzai, an Afghan tribal warlord and alleged Taliban associate imprisoned for life in the United States for smuggling heroin into the country. But the US has expressed little interest in the deal.

Frerichs’s sister, Charlene Cakora, pleaded in an opinion piece Thursday in The Washington Post for Biden to bring him home.

“The US government either makes this trade or it doesn’t save my brother’s life. Every day we don’t bring Mark home is another day he remains in danger,” Cakora wrote.

As the United States completed its withdrawal last August, White House spokeswoman Jen Psaki said the State Department and other US officials were engaged in pushing for the release of Frerichs.

“They’ve continued to press the Taliban for his release, continued to raise the status in senior level engagements,” she said.