Former President Barack Obama's ambassador to the United Nations apologized Monday for the Obama administration not recognizing the century-old massacre of more than a million Armenians as genocide.

Samantha Power issued the apology via Twitter on Armenian Genocide Remembrance Day, which commemorates the mass killings of Armenians at the hands of Ottoman Turks in 1915 and 1916.

According to the U.S. Holocaust Museum, as many as 1.2 million Armenians were killed by Ottoman authorities over those two years.

I am very sorry that, during our time in office, we in the Obama administration did not recognize the #Armenian Genocide — Samantha Power (@SamanthaJPower) April 24, 2017

Back in 2008, then-candidate Obama promised that if he were to become president, he and his administration would officially recognize the killings as genocide.

"I also share with Armenian Americans–so many of whom are descended from genocide survivors–a principled commitment to commemorating and ending genocide. That starts with acknowledging the tragic instances of genocide in world history," a 2008 Obama campaign statement said.

Obama failed to follow through on his promise year after year of his presidency.

The United States has not recognized the mass killings largely because doing so would anger Turkey, a longtime U.S. ally and NATO member.

President Donald Trump has followed in his predecessors' footsteps. In a statement released by the White House on Monday to recognize Armenian Genocide Remembrance Day, the Trump administration did not use the word "genocide."

No mention of "genocide" in Pres statement on Armenian Remembrance Day. Spicer says @POTUS using same language as predecessors. pic.twitter.com/h7elkknvW7 — Mark Knoller (@markknoller) April 24, 2017

"Beginning in 1915, one and a half million Armenians were deported, massacred, or marched to their deaths in the final years of the Ottoman Empire," Trump said in the statement. "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."

Trump referred to the massacre of Armenians as "one of the worst mass atrocities of the 20th century."