Britain is drafting a United Nations Security Council resolution to establish a UN investigation, but Ms Clooney said the Iraqi government needs to send a letter formally requesting the inquiry before the 15-member council can vote.
ISIS is committing genocide against the Yazidis in Syria and Iraq to destroy the minority religious community through killings, sexual slavery and other crimes, UN experts reported in June last year.
Ms Clooney, who represents Nadia Murad and other Yazidi victims of ISIS, said that despite public support by Iraq for a UN investigation, the government has not yet made a request.
'We do want to see an investigation take place with the cooperation of the Iraqi authorities,' Ms Clooney told Reuters in an interview after speaking at a United Nations event on accountability for crimes committed by ISIS.
'But ultimately if that support is not forthcoming in terms of real action, then the UN has to think of other ways in which to achieve accountability,' she said.
The Yazidis are a religious sect whose beliefs combine elements of several ancient Middle Eastern religions. ISIS consider the Yazidis to be devil-worshippers.
Ms Clooney said the Security Council could establish an inquiry without Iraq's consent, the 193-member UN General Assembly could establish a special team to preserve evidence and prepare cases - as it did for Syria in December - or the Security Council could refer the case to the International Criminal Court.
'All these options are on the table. They must be seriously considered, because victims like Nadia can't expect to wait forever,' said Ms Clooney, adding that it was extremely important that evidence was preserved for future prosecution.
When asked what may be preventing Iraq from requesting the investigation, Murad, 23, speaking through a translator, said: 'They think that all Iraqis are persecuted by Daesh and they have to seek justice for everybody.'
'We will be seeking help and assistance,' Iraq's UN ambassador, Mohamed Ali Alhakim, told the UN event where Murad also spoke. He did not immediately respond to a request for comment on why Iraq had not yet requested a UN inquiry.
Britain's UN ambassador, Matthew Rycroft, told the UN event that he was working with Iraq on the issue.
'The proposal should support Iraq's national efforts and fully respect it national laws and sovereignty. But it is also an urgent task. And we look forward to finalizing that proposal with you very, very soon,' he said.
Murad was abducted and held by ISIS for three months in 2014 in Mosul. She told her story to the UN Security Council in December 2015 and since then has been campaigning for justice.
An exhausted Murad told UN ambassadors on Thursday: 'My words, tears and my testimony have not made you act. I wonder whether there is any point in continuing my campaign at all.'
Reported by Michelle Nichols and Edited by Leslie Adler
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)