said last month that he believed Mr. Biden would make the genocide declaration despite knowing that a reset of U.S.-Turkey relations would then become “much harder.”

Mr. Erdogan’s aides have signaled that Mr. Biden’s declaration would face a hostile reaction in Turkey. The foreign minister, Mevlut Cavusoglu, said in a Turkish media interview this week that “if the United States wants to worsen ties, the decision is theirs.”

According to a tally by the Armenian National Institute, a Washington-based group, at least 30 countries have done so.

The answer is more complicated concerning the United Nations, which played a central role in the treaty that made genocide a crime but has not taken a position on what happened in 1915 — 30 years before the global body was created. The website of its Office on Genocide Prevention and the Responsibility to Protect, in describing the origin of the term genocide, does not mention Armenia. António Guterres, the secretary-general, has skirted the issue.

Asked on Thursday about Mr. Guterres’s view, his spokesman, Stéphane Dujarric, said: “We have no comment, as a general rule, on events that took place before the founding of the U.N.” Genocide, Mr. Dujarric said, “needs to be determined by an appropriate judicial body, as far as the U.N. is concerned.”

Lara Jakes contributed reporting.

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