Islamic State jihadists are using rape as a weapon to terrorise and destroy communities, Angelina Jolie has warned.
"They are dictating it as policy... beyond what we have seen before," the UN special envoy told a House of Lords committee on sexual violence in war.
Ex-UK Foreign Secretary William Hague told the committee the use of rape in war should "shame all men" but he said it was possible to combat the problem.
The pair have been campaigning together to end the use of rape in war zones.
'Sold for $40'
Last year they hosted a global summit in London attended by representatives from more than 100 countries aimed at raising awareness and tackling the issue.
Ms Jolie, who is also an actress, appeared before the newly established committee on sexual violence in conflict, as part of its first evidence session.
She shared harrowing stories of girls she had met in war zones who she said had been repeatedly raped and sold for as little as $40 (£26).
At the scene
By BBC reporter Shelley Phelps
It's not often that a Hollywood film star graces the dusty old rooms of the House of Lords, an institution whose average member is aged 70.
So it was no surprise that a queue and a media scrum built up in the corridor outside the room where Angelina Jolie was due to appear.
Chancellor George Osborne happened to stroll past. "I know my place. I'm in the committee next door but I don't expect to see you there," he joked.
Ms Jolie made confident, eloquent and moving contributions during the session, which lasted for just over an hour.
Acknowledging that while she could use her global reach to raise awareness of the issue, art and celebrity had their limits, she told peers and journalists. "Policy needs to change".
Asked what the root causes of the problem were, Ms Jolie said: "I think the most important thing to understand is what it's not. It's not sexual, it is a violent, brutal terrorising weapon.
"Unfortunately it is everywhere, in and out of conflict in every country basically. I can't think of one where there is not this issue."
She said so-called IS was explicitly saying to its fighters: "We ask you to rape.
"They are saying, 'you should do this, this is the way to build a society'."
The actress said the group knows "it is a very effective weapon and they are using it as a centrepoint of their terror and their way of destroying communities and families, and attacking and dehumanising".
Ms Jolie praised the work of people in the field, describing them as her "heroes", but added: "Laws need to change, policies need to change, governments and leaderships need to come together and that will make the real change."
Mr Hague, who stood down as an MP this year, said sex crimes were committed during conflicts to make peace and reconciliation more difficult to achieve and to create greater flows of refugees out of conflict areas.
Combating the problem, therefore, had to form a key part of any successful foreign policy, and not be simply an "add on", he argued.
He added: "Often I have been asked why is a man pursuing this subject; a breathtaking question when you think about it. But you do get asked that.
"But these are crimes that are committed almost exclusively by men - and that they happen and have happened for many years and go unchallenged should shame all men.
"Men and male world leaders have a crucial role to play in tackling this issue."