Members of the Yazidi community spoke of their anguish at the plight of family members in northern Iraq as they held a demonstration to urge the UK government to help their people.
Around 250 men, women and children from across the Kurdish diaspora and now living in the UK converged on Downing Street to hand letters to Prime Minister David Cameron, calling on him to help stop the massacre by IS - formerly known as ISIS.
Among them were Yazidis living in the UK, who spoke with horror of family members being killed and others "dying slowly" while they are trapped on Mount Sinjar in northern Iraq.
Thousands of Yazidi people have been forced to flee to the mountain to escape slaughter by Islamic extremists, and now face starvation and dehydration on the moutainside.
Defence Secretary Michael Fallon said Britain was ready to provide "technical assistance" to support US humanitarian operations in the region and that he hoped British air-drops for the trapped Yazidi minority community could begin in the "next couple of days".
Kurds from Iraq, Iran, Syria and Turkey joined together in London to call for more action from the UK, US and the United Nations.
Among the Yazidis demonstrating was Dr Khairi Al Shareef, 53, an engineering researcher from Cardiff who has lived in the UK since 1990.
Speaking through tears, he said his family had been forced to flee the Yazidi town of Bashiqa after IS militants moved in.
He said: "There is nobody left, Christian or Yazidi, any minorities - 500,000 families have fled. Isis came yesterday and they said 'If you don't convert to Islam within three days, we will chop off your heads according to Sharia law.'
"I have family there, I haven't heard from them. They tried to go to Turkey yesterday but they couldn't get in because they didn't have passports because they fled without anything and left all their belongings behind."
Dr Al Shareef said he had around 50 brothers and sisters in danger and that he did not know where they were.
"We cry. We can't do anything. They are defenceless. I am very worried about my family," he said. "There are 40,000 people in imminent danger. They are trapped on the mountain and they are only giving them water by spoons, and Isis are below them.
"We have heard that they have taken 500 women away, killed men and taken beautiful or young women. They are in danger of imminent genocide or death."
Holding out a letter he hoped to pass to Mr Cameron, he said: "Please save us. Please save the Yazidi."
Arkin Abdi, 41, lived in Sinjar but said he moved to the UK 10 years ago to flee terrorism from Muslims.
Mr Abdi, also Yazidi, said his uncle and his uncle's children were among 30 family members killed, while his cousin and 50 other relatives had been trapped on Mount Sinjar for five days with around 4,000 other people.
"I called him last night about 2 o'clock and he said everyone was dying slowly because no-one was coming to help. I am very sad. I don't know who to cry for - my family or the Yazidi people."
Mr Abdi said he had heard reports of militants killing men and taking women away to the northern Iraqi city of Mosul, where they were being sold for four dollars each.
Zyad Haji, a 38-year-old PhD student at the University of Manchester who was at the demonstration in London, also said he had lost contact with his family.
He said the last time he spoke to his father and older sister, Mayan, was on August 3 when they were both still at home.
But relatives who managed to escape from Mount Sinjar have told Mr Haji that Mayan is still trapped there and cannot find her two daughters.
"Our girls and our women, most of them are being raped and assaulted. All this is happening because we're not Muslim. In their view they consider us infidels and worshippers of the devil," he said, adding: "They don't understand us."
"I can't go back because I guarantee you I would be slaughtered. Our properties have been looted and burned."
Ali Saleem Shemo, a Yazidi law student at the University of Derby, said he has also lost touch with his six brothers and sisters, several of whom are children.
He said: "I last spoke to them three days ago, then their (phone) battery died. I have no idea whether they are alive or dead."
The protest in pictures
Mr Shemo's student visa expires in 20 days, and now he is worried about his future.
He said: "I want to go back home but I don't know where to go. I want to be with my family. All Yazidis in Kurdistan have escaped from their homes. We are in a catastrophe, we are in a disaster."
Arzu Pesmen, co-chairman of the Kurdish People's Assembly, which helped organise the demonstration, said: "It is a religious massacre because they are a minority. People have been forced for many, many years to convert to Islam.
"We are trying to get the international public to be aware of the issues and especially the UK and the UN to take action against Isis.
"The US and UK messed the Middle East up and created a lot of problems in the region. They need to be responsible to calm the region down."
Yazidi Refugees In The Sinjar Mountains