"My purpose here today is to assert in my judgment, (ISIS) is responsible for genocide against groups in areas under its control including Yazidis, Christians and Shiite Muslims," he said, during a news conference at the State Department.
Kerry said that in 2014, ISIS trapped Yazidis, killed them, enslaved thousands of Yazidi women and girls, "selling them at auction, raping them at will
and destroying the communities in which they had lived for countless generations," executed Christians "solely for their faith" and also "forced Christian women and girls into slavery."
"Without our intervention, it is clear that those people would have been slaughtered," he said.
This is the first time that the United States has declared a genocide since Darfur in 2004.
The House of Representatives on Monday unanimously passed a resolution labeling the ISIS atrocities against Christian groups in Syria and Iraq "genocide," a term the State Department had been reluctant to use about the attacks and mass murders by the terror group.
finding does not legally obligate the U.S. to take any particular action, but it could put pressure on the Obama administration to take more aggressive military action against ISIS. It could also give weight to calls by other lawmakers and humanitarian groups pushing the Obama administration to welcome more refugees into the United States.
The move, aimed at ramping up pressure on the Obama administration, appears to have worked.
The measure was non-binding, but both Republicans and Democrats in the House joined together 393-0 to back a "sense of Congress" saying the crimes committed against Christians, Yazidis and other ethnic and religious minorities in the region amount to war crimes and, in some cases, genocide.
Republican Rep. Jeff Fortenberry, whose Nebraska district is home to the largest group of resettled Yazidis in the U.S., authored the resolution with California Democratic Rep. Anna Eshoo.
During debate on Monday, Fortenberry noted it was a rare instance of an issue that has "risen above the petty and difficult differences we often work out on the floor of the House of Representatives."
Under a deadline set by Congress, the State Department had until Thursday to formally to decide whether it would issue a comprehensive genocide designation.
Kerry, though, had previously alluded to the possibility that the actions of ISIS, also know as ISIL, were genocide.
"ISIL's campaign of terror against the innocent, including the Yazidi and Christian minorities, and its grotesque targeted acts of violence, show all the warning signs of genocide," Kerry said in August 2014. "For anyone who needed a wakeup call, this is it."
Fortenberry praised the State Department for its decision Thursday.
"I commend Secretary Kerry and the State Department for making this important designation. The genocide against Christians, Yazidis and others is not only a grave injustice to theses ancient faith communities -- it is an assault on human dignity and an attack on civilization itself," he said. "The United States has now spoken with clarity and moral authority."
But another congressional voice, Florida Sen. Marco Rubio
, criticized the administration for not taking action sooner.
"That it took so long for the administration to arrive at this conclusion, in the face of unspeakable human suffering, defies explanation," Rubio, who until Tuesday was a GOP presidential candidate, said in a statement. "At long last the United States is no longer silent in the face of this evil, but it would be travesty if we were to mistakenly take solace in this designation, if the designation did not then yield some sort of action."
Mirza Dinnayi, an Iraqi Yazidi activist who has been helping evacuate former Yazidi slaves of ISIS
to be rehabilitated in Germany, welcomed Kerry's announcement.
"I am very happy to hear that (the U.S.) will recognize the genocide of Yezidi and Christian minorities," he told CNN in an email. "This is an important step to stop the suffering of the persecuted people under the control of the extremist islamic groups, specially ISIS. And this is also important for my community to trust the international community again, because we were left in the hands of Islamic State."
He called on the State Department to push the U.N. to establish an international criminal court case on genocide against the Yazidis and Christians in Iraq and Syria.
"Furthermore," he said, "it is necessary to give the minorities more support to be sure that (these) crimes will not happen again."
A variety of activist and advocacy groups praised Kerry's move Thursday.
An international center advocating against hate, terrorism and anti-Semitism was one to join the chorus.
"The Simon Wiesenthal Center applauds Secretary Kerry's acknowledgement that Christians and Yazidis are targets of Genocide," the organization said in a statement. "We reiterate our call that the U.S put these two groups at the front of the line for consideration for immigration to our country and to redouble our efforts to destroy ISIS."
In Defense of Christians, a group that has heavily lobbied for recognizing what is happening as genocide, put out a statement from its president Toufic Baaklini.
"IDC extends our deepest gratitude to Secretary Kerry and to the Obama administration for carefully reviewing the overwhelming evidence of the genocide against Christians, Yazidis, Shia Muslims and other religious minorities and for proclaiming the irrefutable truth that the crimes they have suffered constitute genocide," Baaklini said.
And the Archbishop of Washington, Cardinal Donald Wuerl, put out a statement of "appreciation."
"For some time, the world has witnessed the deliberate and organized effort by ISIS to eliminate Christians from the Middle East. For the U.S. government to call this savagery by its proper name -- genocide -- is a welcome step in what must now be a more committed effort at bringing peace and security to that beleaguered land," Wuerl said. "These words must now be translated into action."