President Trump paid tribute Monday to more than one million Armenians killed during the Ottoman Empire, but followed his predecessors in stopping short of calling the century-old killings a genocide.

Mr. Trump called the deaths “one of the worst mass atrocities of the 20th century,” with about 1.5 million Armenians “deported, massacred, or marched to their deaths” beginning in 1915.

“I join the Armenian community in America and around the world in mourning the loss of innocent lives and the suffering endured by so many,” the president said in a statement. “We must remember atrocities to prevent them from occurring again.”

President Barack Obama had pledged during his campaign in 2008 to become the first U.S. president to recognize the episode as a genocide. But Mr. Obama failed to do so annually as president, deciding against provoking Turkey, a key NATO ally that has rejected such a characterization of the killings.

Presidents George W. Bush and Bill Clinton also avoided the term “genocide” after pledging during their campaigns to recognize it as such.

Mr. Trump didn’t make such a campaign pledge. He said Monday that the U.S. welcomes “the efforts of Turks and Armenians to acknowledge and reckon with painful history, which is a critical step toward building a foundation for a more just and tolerant future.”

“As we reflect on this dark chapter of human history, we also recognize the resilience of the Armenian people,” Mr. Trump said. “Many built new lives in the United States and made indelible contributions to our country, while cherishing memories of the historic homeland in which their ancestors established one of the great civilizations of antiquity.”

Armenian National Committee of America Executive Director Aram Hamparian said Mr. Trump “has chosen to enforce Ankara’s gag-rule against American condemnation and commemoration of the Armenian genocide.”

“In failing to properly mark April 24th, President Trump is effectively outsourcing U.S. genocide-prevention policy to [Turkish President] Recep Erdogan, an arrogant and authoritarian dictator who clearly enjoys the public spectacle of arm-twisting American presidents into silence on Turkey’s mass murder of millions of Armenians, Greeks, Assyrians, and other Christians,” Mr. Hamparian said.