A city MP has revealed he knows of three Birmingham men who have been arrested after travelling to fight in Syria and Iraq.
Khalid Mahmood, Labour member for Perry Barr, added that up to 2,000 British residents are thought to have travelled to the warzone.
Mr Mahmood said that, in one case, the young man was arrested on his return after fighting for nine months in the Middle East after the student's mother contacted police.
He was speaking after Foreign Secretary William Hague told the Commons the Government believed up to 400 British residents might be fighting in Syria.
Mr Hague warned they could pose a risk to the UK once they returned.
Earlier this month, a video posted online by militant group the Islamic State in Iraq and the Levant or Isil (also sometimes called Isis) appeared to show British and Australian nationals urging Muslims to join them in holy war in Iraq.
Mr Mahmood said he believed the number was "certainly 1,500 and possibly 2,000 or more."
He said: "If you look across the country, you look across Birmingham and you look at what's gone on, it's a high number.
"The first wave of people that went across included British Syrians and British Kurds. The original so-called freedom fighters who went were people who had families in Syria.
"And then they were followed by British Muslims - Asian Muslims predominantly and possibly Somalian Muslims too."
The MP said he knew of three Birmingham men who had joined a group called Jabhat al-Nusra, which is connected to Al-Qaeda.
"Two of my constituents at the moment are held up in prison. They are two lads who went from Birmingham who were at Birmingham City University, radicalised within six months and went to join al-Nusra and they came back - there was a third lad with them from Sheldon - and they have all been arrested.
"This was just before December when they came back after having spent nine months there because one of the lads left a note to his mum saying 'I am going to join al-Nusra'.
"So she called the police and told them what was going on. They were able to pick him up when he came back.
"Once people have been radicalised, and are used to using weapons and resolving situations using live ammunition and weapons, I think it is difficult to say them 'Alright, you can come back and it will be okay'."
A spokesman for the Foreign Office said they did not have figures for numbers of people arrested on return to the UK from the Middle East conflict.
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