The United States President Joe Biden urged Afghanistan's Taliban to release a U.S. civil engineer who was abducted two years ago and is believed to be the last American hostage held by the Taliban.

Mark Frerichs, a 59-year-old U.S. Navy veteran from Lombard, Illinois, worked in Afghanistan for a decade on development projects. He was kidnapped a month before the February 2020 U.S. troop pullout deal was signed and was transferred to the Haqqani network, a brutal Taliban faction accused of some of the deadliest attacks of the war.

Monday marks his second year in captivity. "Threatening the safety of Americans or any innocent civilians is always unacceptable, and hostage-taking is an act of particular cruelty and cowardice," Biden said in a statement.

"The Taliban must immediately release Mark before it can expect any consideration of its aspirations for legitimacy. This is not negotiable."

Biden pulled U.S. troops out of Afghanistan in August in a chaotic withdrawal that drew sharp criticism from Republicans and his own Democrats as well as foreign allies and punctured his approval ratings.

Frerichs' family has criticized the U.S. government for not pressing harder to secure his release. Last week, his sister, Charlene Cakora, made a personal plea to Biden in a Washington Post opinion piece titled, "President Biden, please bring home my brother, the last American held hostage in Afghanistan."