By Arshad Mohammed

WASHINGTON (Reuters) - Islamic State has committed genocide against Christians, Yazidis and Shi'ite Muslims, the United States said on Thursday, a finding U.S. officials hope will bring more resources to help the groups even though it does not change U.S. military strategy or legal obligations.

"In my judgment, Daesh is responsible for genocide against groups in areas under its control, including Yazidis, Christians and Shi'ite Muslims," U.S. Secretary of State John Kerry told reporters, referring to the group by an Arabic acronym. "Daesh is genocidal by self-proclamation, by ideology, and by actions."

Republicans, who control the U.S. Congress, had pressured the Democratic White House to call the militants' atrocities in Iraq and Syria genocide and the House of Representatives on Monday passed a nonbinding resolution 393-0 labeling them as such.

U.S. officials hope the determination will help them win political and budget support from Congress and other nations to help the targeted groups return home if and when Islamic State-controlled areas such as the Iraqi city of Mosul are liberated.

While the genocide finding may make it easier for Washington to argue for greater action against the group, U.S. officials said it does not create a U.S. legal obligation to do more, and would not change U.S. military strategy toward the militants.

On Wednesday, State Department spokesman Mark Toner said: "Acknowledging that genocide or crimes against humanity have taken place in another country would not necessarily result in any particular legal obligation for the United States."

U.S. President Barack Obama ordered air strikes against the group starting in 2014 but has made clear he wishes to avoid any large commitment of U.S. ground troops.

Unlike in Rwanda in 1994 and Darfur in 2004, where the United States found that genocide had taken place but did not use military force to stop it, U.S. officials noted they began air strikes against Islamic State targets in Iraq in August 2014 in part to save the Yazidi minority group from targeted attack.

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"We didn’t act in Rwanda. We looked back and regretted it. We didn’t act militarily in Darfur. In this case within ... days of the Yazidis being targeted by Daesh in Iraq, American planes were in the air trying to help them," said a senior U.S. official who spoke on condition of anonymity.

'WE'VE DONE AN ENORMOUS AMOUNT'

Islamic State militants have swept through Iraq and Syria in recent years, seizing swathes of territory with an eye toward establishing jihadism in the heart of the Arab world.

The group's videos depict the violent deaths of people who stand in its way. Opponents have been beheaded, shot dead, blown up with fuses attached to their necks and drowned in cages in swimming pools, with underwater cameras capturing their agony.

Kerry argued the United States has done much to fight the group since 2014, but did not directly answer a question on why it had not done more to prevent genocide.

"We're very confident we've done an enormous amount," he told reporters as he walked down a hall at the State Department.

"The fact is that Daesh kills Christians because they are Christians. Yazidis because they are Yazidis. Shi'ites because they are Shi'ites," Kerry said earlier, and accused Islamic State of crimes against humanity and of ethnic cleansing.

Islamic State militants have exploited the five-year civil war in Syria to seize areas in that country and in neighboring Iraq, though U.S. officials say their air strikes have markedly reduced the territory the group controls.

On-again, off-again peace talks got under way this week in Geneva in an effort to end the civil war, in which at least 250,000 people have died and millions have fled their homes. A fragile "cessation of hostilities" has reduced, but not ended, the violence over the last two weeks.

(Additional reporting by Patricia Zengerle; Editing by James Dalgleish)