ANKARA, April 25 (Reuters) -U.S. President Joe Biden's declaration that massacres of Armenians in the Ottoman Empire constituted genocide is "simply outrageous" and Turkey will respond over coming months, Turkey's presidential spokesman said on Sunday.

Biden broke on Saturday with decades of carefully calibrated White House comments over the 1915 killings, delighting Armenia and its diaspora but further straining ties between Washington and Ankara, both members of the NATO military alliance.

"There will be a reaction of different forms and kinds and degrees in coming days and months," Ibrahim Kalin, President Tayyip Erdogan's spokesman and adviser, told Reuters in an interview.

Kalin did not specify whether Ankara would restrict U.S. access to the Incirlik air base in southern Turkey, which has been used to support the international coalition fighting Islamic State in Syria and Iraq, among measures it may take.

After other Turkish officials swiftly condemned Biden's statement on Saturday, Erdogan would address the issue after a cabinet meeting on Monday, Kalin said. "At a time and place that we consider to be appropriate, we will continue to respond to this very unfortunate, unfair statement," he said.

Turkey accepts that many Armenians living in the Ottoman Empire were killed in clashes with Ottoman forces in World War One, but denies the killings were systematically orchestrated and constitute genocide.

For decades, measures recognising the Armenian genocide stalled in the U.S. Congress and most U.S. presidents have refrained from calling it that, held back by concerns about straining relations with Turkey.

But those relations are already troubled. Washington imposed sanctions on Turkey over its purchase of Russian air defences, while Ankara has been angered that the United States has armed Kurdish YPG fighters in Syria and not extradited a U.S.-based cleric Turkey accuses of orchestrating a 2016 coup attempt.