WASHINGTON -- President Barack Obama is marking the anniversary of the massacre of Armenians in Turkey nearly a century ago by calling it a "one of the worst atrocities of the 20th century." But as in previous years, he is stopping short of calling the killings genocide.

As a candidate for president, Obama vowed to recognize the genocide once in office.

But since his election, Obama has declined to use the word in the face of furious resistance from Turkey, a key NATO ally.

Most historians see the killings as the first genocide of the 20th century. They estimate that about 1.5 million Armenians died. However, Turkish leaders have long rejected the term. They say that the figures are inflated and that there were deaths on both sides as the Ottoman Empire collapsed.

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Thousands of demonstrators march to mark the death of 1.5 million Armenians in the former Ottoman empire, in Los Angeles Tuesday, April 24, 2012.

The demonstration is an annual remembrance of the 1915 killings of Armenians in the Turkish territories during World War I.

Armenians refer to the killings as genocide, while Turkey contends there was no organized slaughter.

(AP Photo/Damian Dovarganes)