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The United States declared Thursday that the Islamic State group's slaughter of Christians, Yazidis and Shias in Iraq and Syria amounts to a genocide.

Secretary of State John Kerry's "moral statement" does not place the United States under any new legal obligations, State Department officials said.

But Washington will work hard to collect and record evidence and share it with its international partners as states seek a way to bring justice to bear.

"Daesh is genocidal by self-proclamation, by ideology and by actions, in what it says, what it believes and what it does," Kerry said, using an Arabic acronym for the IS group.

"Daesh is also responsible for crimes against humanity and ethnic cleansing directed at these same groups," he added.

The Islamic State group recruits Sunni extremists and has regularly carried out mass killings of Shia Muslim, Christian and Yazidi prisoners.

In June 2014, it seized the formerly cosmopolitan city of Mosul in northern Iraq, placing whole communities under threat of murder, rape or enslavement.

A few days later the group killed at least 1,700 Shia army cadets as they fled Camp Speicher, a former US military base just north of Saddam Hussein's home town of Tikrit.

In March last year, UN investigators warned the self-proclaimed caliphate was trying to wipe out Iraqi Yazidis, members of a pre-Islamic religious minority.

"In my judgment, Daesh is responsible for genocide against groups in areas under its control including Yazidis, Christians and Shia Muslims," said the US Secretary of State.

"For those communities, the stakes in this campaign are utterly existential," he said referring to the international coalition campaign against IS in Iraq and Syria.

Kerry issued his ruling after the US Congress voted to declare the killings a genocide and demanded the administration take a position.