Photo: BBC NEWS

Two men aged 29 and 18 from Levenshulme in Manchester, and a 21-year-old woman from Trafford are in custody.

A 29-year-old man from Oxford has also been arrested on being concerned in the commission, preparation or instigation of acts of terrorism, say police.

A number of properties are now being searched.

All four are being questioned in Manchester by the North West Counter-Terrorism Unit (NWCTU).

Detective Chief Superintendent Tony Mole, head of NWCTU, said the operation had been running since autumn 2013 "since we first became aware of a number of individuals travelling from the North West to the battlefields of Syria".

"Like other major cities across the UK we have been aware for some time of people travelling to war-torn places for terrorism-related purposes. This is not a problem unique to Manchester or the North West - it is an issue that affects different communities across the country and elsewhere.

"We look at every case on its own merits, but travelling to a warzone in order to be involved in conflict, or supporting those who do so, could make you potentially liable to prosecution for terrorism offences.

"Also, as well as posing a risk to themselves, there is the very real threat that they could pose a danger to our own communities when they return to the UK."