Britons travelling to Syria and Iraq to join Islamic State militants face a "life of war, famine and hardship", Home Secretary Theresa May has warned.
She made the direct appeal to those "tempted" to travel at an international counter-terrorism conference in London.
It comes as three Bradford sisters are feared to have travelled to Syria with their nine children.
Their "devastated" parents have issued a statement urging anyone considering making such a journey "not to go".
Efforts are continuing to trace Khadija, Sugra and Zohra Dawood and their children, who disappeared after travelling to Saudi Arabia to take part in a religious pilgrimage.
They took a flight to Istanbul and there are indications they have now crossed the border into Syria.
The women's parents and other relatives said on Thursday that they were "devastated" and in "great distress".
"We are very worried about the children who could now be in a dangerous place," they said.
"We do not support the actions of the sisters leaving their husbands and families in the UK and of taking their children into a war zone where life is not safe to join any group.
"We plea to anyone thinking about a similar journey not to go."
The women's brother, Ahmed Dawood, is believed to be fighting with extremists in Syria, parts of which are controlled by IS militants.
The conference, organised by Met Police Commissioner Sir Bernard Hogan-Howe, also comes after Talha Asmal, 17, from Dewsbury, West Yorkshire, is thought to have become the UK's youngest-ever suicide bomber.
The home secretary said: "In the past week we have heard separate and equally shocking media reports about dangerous radicalisation.
"Some of these cases have not been confirmed. But while I cannot comment in detail I do want to say this to the parents, siblings, faith leaders and community groups whose loved ones may be at risk: I want to work with you to stop this from happening."
"A real partnership" is needed to defeat the "poisonous" and "twisted" ideology of IS, Mrs May said at the event, organised following terrorist attacks around the world, including in Paris and Sydney.
At least 700 people from the UK are believed to have travelled to Syria or Iraq to support or fight for jihadist organisations. About half have since returned.
Mrs May said there was a "very serious threat that returnees may carry out attacks on home soil, radicalise others or fund or facilitate terrorism in other ways."
Groups like Boko Haram and al-Qaeda also posed a danger, she said, and "the threat from Northern Ireland-related terrorism and far-right extremism" should not be forgotten.
'Turning backs on families'
The home secretary said IS wanted to "lure" young women and families to Syria and Iraq, through the "powerful allure of propaganda".
Appealing to those thinking of travelling, Mrs May said they would be going to a place with "no democracy, no rule of law, no equality".
She said: "If you travel, you reject those freedoms. You turn your back on your families and your communities and instead embrace hatred, intolerance and brutality."
Mrs May added: "Do not travel. Do not engage in dangerous activities."
Speaking to the police from 20 countries and 100 law enforcement officials attending the event, she warned: "If you do not follow this advice, if you become involved in illegal and harmful activities, we will do everything in our power to keep the people of this country safe from terrorism."
Sir Bernard, speaking before the conference, said there was a "growing phenomenon" of British people taking their families to Syria.
The commissioner said: "How could anybody want to go to a war zone? Why would you take a child there? This is a worry.
"We're just struggling to understand what drives people to do this."
He said more research was needed to understand why people were attracted to the Islamic State regime that most people regarded as "barbaric".
The commissioner welcomed recent legal changes which allow police to seize the passports for 28 days of people they suspected might be travelling to engage in fighting.
Mrs May told the conference that the new powers of temporary seizure had been used and "proved effective".