British Muslim families should act if they suspect their children or siblings are thinking of travelling to join conflicts in Syria and Iraq, the Home Secretary has said.

Theresa May today backed a nationwide campaign by not-for-profit organisation Families Against Stress and Trauma (Fast), which puts the emphasis on families preventing their loved ones from travelling to join the conflicts.

Speaking at the Families Matter launch in London, she said: "I would encourage all families to see the answer lying with them and their communities."

Several high-profile incidents have led to a national effort to prevent young people travelling to Iraq and Syria.

Last month two men from Cardiff and a man from Aberdeen featured in an online recruitment video urging western Muslims to join the fighting with the Islamic State in Iraq and Greater Syria (Isis) group.

Nasser Muthana, 20, Reyaad Khan, 20, and Abdul Raqib Amin, 26, appeared in the video and are thought to be among more than 400 Britons fighting in Syria and Iraq.

Nasser's father Ahmed, who appeared in a Fast campaign video and whose other son Aseel is also in Syria, said: "Nasser was going to be a GP. We expected a high life for him. He was the best of the best but unfortunately he chose to go with these wrong people.

"I think 'am I going to see him alive again?' Maybe we won't even see the coffin - we'll just see on the news they're dead."

The Home Office has stepped up measures to act as a deterrent, including prosecuting some of those who come back as terrorists.

Ms May, who said the UK Government has contributed £600 million in humanitarian aid to the Syria crisis, added: "I am clear, in relation to people who are going to Syria and returning as terrorists, the Government will take action.

"Over 400 UK-linked individuals have now travelled to Syria since the uprising began. Anyone travelling to Syria and Iraq is exposing themselves to serious risks and the Government strongly advises against it.

"Providing a campaign called Families Matter is about supporting the families. Remember the family and the stress and trauma it causes when someone travels to Syria. The message is don't go to Syria - there are better ways to help.

"The best way people can help respond to the crisis is from the UK within the UK and within the safety of their families and communities.

The campaign also comes after Manchester twins Salma and Zahra Halane, 16, who have 28 GCSEs between them, disappeared from their family home last month when they flew out to Turkey and are believed to have crossed the border.

It is understood an older brother from the large Somalian family has joined the extremist militant group Isis, and one line of inquiry is that the girls may have travelled to join him.

Fast founder Saleha Jaffer said Muslim families were particularly concerned about the access to social media and influence of siblings on young people, although she backed prosecutions for those who become radicalised.

She said: "It is hard to properly convey the anguish of those left behind. Families matter because they are torn apart by those that choose to travel but they have the power to make a real difference.

"A lot of families are worried about social media and often children don't even tell their families they are going away - that's the biggest problem. We are asking them to look out for any small tell-tale signs.

"I think if a brother has gone then the families need to concentrate on the siblings and understand what they are thinking. They don't need to tell them off, they need to be friends.

"I don't think they are going to criminalise everyone but I think if the Government has intelligence that they are radicalised or extremists then yes I agree they should be prosecuted."

To see the campaign video visit www.familiesmatter.org.uk.