US lawmakers voted unanimously last night to approve a motion which declared ISIS' treatment of Christians and other religious minorities is "genocide".

The House of Representatives voted by 393 to zero that "the atrocities perpetrated by ISIL against Christians, Yezidis, and other religious and ethnic minorities in Iraq and Syria constitute war crimes, crimes against humanity, and genocide."

Rep. Jeff Fortenberry, who tabled the motion, said: "When ISIS systematically targets Christians, Yezidis, and other ethnic and religious minorities for extermination, this is not only a grave injustice – it is a threat to civilization itself.

"We must call the violence by its proper name: genocide."

He was supported by the Republican leadership in the House as Speaker Paul Ryan and House Majority Leader Kevin McCarthy backed the motion.

The vote will put further pressure on the White House to declare a genocide. If Secretary of State John Kerry took the unusual move of declaring an ongoing conflict a genocide, it would place significant obligations on the US to step up its intervention in the Middle East.

When asked why the administration has yet to use the term, White House spokesperson Josh Earnest said the word "involves a very specific legal determination that has, at this point, not been reached."

However ahead of the vote, State Department spokesman John Kirby said he did not expect any resolution voted on in the House to be a factor in the decision.

Most of ISIS' opponents are Muslim but the militant group has been accused of particularly targeting religious minorities, including Yazidis and Christians.

The European Parliament has already passed a motion which labelled the atrocities a genocide but the UK Parliament has yet to do so, despite significant pressure from parliamentarians and campaign groups.

In addition to the vote on genocide, on Monday the House also voted by 392-3 to endorse a separate resolution which accused Syrian President Bashar al-Assad of war crimes. The motion declared the House "strongly condemns the continued use of unlawful and indiscriminate violence against civilian population by the Government of Syria, its allies, and other parties to the conflict."

Monday's vote came after the release of a detailed report which documented the persecution faced by Christians in Syria and Iraq at the hands of ISIS. The 278-page report from Knights of Columbus and In Defence of Christians was compiled after fears were raised Kerry would declare crimes against Yazidis a genocide but not those against Christians.

The State Department has until Thursday to formally decide whether ISIS' crimes amount to genocide but it is believed the deadline imposed by Congress will be missed. However Steve Oshana, director of campaign group Demand for Action, told Christian Today he believed the Obama administration had already made a decision.

"If I were to guess I think they have already made their determination at this point and it is just a question of when they will come out with it," he said.

"I cannot imagine there are still deliberations given all the evidence that has already come out."