ISTANBUL — Turkey’s president said Monday he was “highly saddened” by U.S. President Joe Biden’s decision to mark as genocide the mass deportations and massacres of Armenians in the early 20th century Ottoman Empire, calling it baseless and unjust.

Recep Tayyip Erdogan said Biden’s decision undermined Turkish-American relations but expressed hope they would discuss that and other issues during a first meeting in years at the June NATO summit and “open the doors of a new period.”

Otherwise, Erdogan said, Turkey would have to take necessary steps but did not elaborate.

On Saturday, Biden followed through on a campaign promise to recognize the events that began in 1915 and killed an estimated 1.5 million Armenians in the Ottoman Empire as genocide.

“We believe the expressions in the statement were included with the pressure of radical Armenian and anti-Turkish groups, but this situation does not resolve the destructive effects it has on bilateral relations,” Erdogan said.

In his speech Monday, the Turkish president spoke at length about the events, that started during WWI. Erdogan said that Armenian rebels killed many defenseless Turkish civilians, since the men of military age were all away fighting.

Erdogan emphasized that historians should be studying the events and that the term “genocide” is too sensitive to be left to presidents or parliaments. He said the designation came into effect after 1948 and must be based on evidence and court decisions.

Earlier Monday, a small group of demonstrators gathered outside the American consulate in Istanbul to protest Biden’s decision.