Theresa May has appealed to Britons who consider joining extremists to fight in Iraq and Syria, telling them: "Do not travel."
The Home Secretary also addressed the families of those at risk of being lured into extremism following reports of the death of Britain's youngest suicide bomber and fears that three sisters have taken their nine children to join extremists.
Opening a counter-terrorism conference in London, she 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."
The Home Secretary said authorities are seeing what they are up against "time and again".
She continued: "The powerful allure of propaganda pumped out by Isil and others to recruit and brainwash British men and women.
"The access social media and modern communications gives terrorists to vulnerable people and the desire of those terrorists to poison others against our values and our way of life."
Mrs May was addressing more than 100 senior police representatives, including delegates from countries around the world at an event to discuss the terrorist threat hosted by the Metropolitan Polce.
She concluded by saying she wanted to "issue a direct message to those British men and women who are thinking about travelling to fight and live in Syria and Iraq".
"I know you've been told that fighting for Isil represents a glorious cause," she said. "I know you've heard that Syria offers your family a new way of life.
"Yes, I recognise that some of you may have been tempted by these messages on social media, the internet and elsewhere.
"But to you I say this: travelling to Syria and Iraq is dangerous. You will be putting your life and the lives of your families at serious risk.
"You will be subjecting yourselves and your children to a life of war, famine and hardship.
"You will be travelling to a place where there are none of the freedoms and opportunities which you have enjoyed in this country. There is no democracy, no rule of law.
"If you travel, you reject these freedoms. You turn your back on your families and your communities and instead you embrace hatred, intolerance and brutality."
She said her message was clear: "Do not travel. Do not engage in dangerous activities. Do not put yourself or those you care about at risk."
Mrs May finished her speech with a warning to those who flee to join extremist groups.
"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."