More than a century after the Ottoman Empire’s killing of an estimated 1.5 million Armenian civilians, President Biden is preparing to declare that the atrocities were an act of genocide, according to officials familiar with the internal debate.

Mr. Biden is expected to announce the symbolic designation on Saturday, the 106th anniversary of the beginning of what historians call a yearslong and systematic death march that the predecessors of modern Turkey started during World War I. He would be the first sitting American president to do so, although Ronald Reagan made a glancing reference to the Armenian genocide in a 1981 written statement about the Holocaust, and both the House and the Senate approved measures in 2019 to make its recognition a formal matter of U.S. foreign policy.

A U.S. official with knowledge of the administration’s discussions said Mr. Biden had decided to issue the declaration, and others across the government and in foreign embassies said it was widely expected.

Jen Psaki, the White House press secretary, declined to comment on Wednesday except to note that the administration would have ''more to say'' on the topic on Saturday.