Prime Minister Justin Trudeau and the majority of Liberal MPs have voted against a Conservative motion declaring that the violence perpetrated by the Islamic State of Iraq and the Levant constitutes genocide.

"Canada strongly condemns the atrocities committed by (ISIS) in the Middle East, and we stand with our allies in the fight against (ISIS) to make sure they lose the capacity to take so many civilian lives," Trudeau said during question period Tuesday when pressed on why he would not vote for the motion.

The prime minister said the Liberal government has instead formally asked the UN Security Council to decide on whether calling it genocide is appropriate.

"We do not feel that politicians should be weighing in on this first and foremost," Trudeau said.

"Determinations of genocide need to be made in an objective, responsible way. That is exactly what we have formally requested the international authorities weigh in on."

Interim Conservative leader Rona Ambrose's motion accused ISIS of crimes against humanity, targeting groups such as Christians, Yezidis, Shia Muslims and other religious and ethnic minorities in Syria and Iraq.

The motion also asked MPs to agree that ISIS is using sexual violence as a weapon of war, enslaving women and girls and targeting gay and lesbian people for murder and torture.

The motion also called on MPs to condemn these atrocities and declare them to be genocide.

On Tuesday, Ambrose responded to Trudeau by accusing the Liberals of tarnishing the Canadian reputation as a protector of human rights.

"This is a low point for the Liberal party and it is a dark spot on Canada's record as a defender of human rights," Ambrose said during question period.

Four Liberal MPs vote with opposition

The motion did not pass, with 166 MPs voting against it — including the majority of Liberal MPs and Green party Leader Elizabeth May — while 139, mostly Conservatives and New Democrats, voted for it.

However, four Liberal backbenchers decided to support the motion: Nathaniel Erskine-Smith, Anthony Housefather, Karen Ludwig and Borys Wrzesnewskyj.

Housefather noted it was a free vote, saying he made his decision based on what he and his constituents believe.

"I believe that ISIS is committing horrible crimes, and I personally believe it is a genocide," said the Montreal-area MP.

Housefather also said he did not consider the motion to be a Conservative one, even though Ambrose was the one to put it forward.

"I believe that motions that come before the House should be considered by everybody as members representing their ridings, and we should not be considering what party put them forward."