Police have arrested four people in Manchester and Oxford suspected of being involved in terrorism-related offences linked to Syria.

The arrests on Tuesday are the latest in a series as concerns mount over the number of Britons travelling to the war-torn Middle East country.

Officers from the north-west counter-terrorism unit (NWCTU) arrested two men aged 29 and 18 in Levenshulme, Manchester, and a 21-year-old woman in Trafford, Greater Manchester. A 29-year-old man in Oxford was detained by the south-east counter terrorism unit.

The four arrested are in custody in Manchester and being questioned by officers from the NWCTU on suspicion of being involved in travelling to or supporting fighting in war zones. Police said a number of properties were being searched.

The head of the NWCTU, Detective Chief Superintendent Tony Mole, said: "The operation has 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 … We look at every case on its own merits, but travelling to a war zone in order to be involved in conflict, or supporting those who do so, could make you potentially liable to prosecution for terrorism offences. 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." He said that there were concerns that people travelling to Syria, whether for humanitarian reasons or to support the Syria opposition, may be targeted by extremist groups who want to recruit them.

Last month, counter-terrorism police searched the Manchester home of 20-year-old Anil Khalil Raoufi, who is believed to have been killed fighting in Syria. It is estimated that about 250 Britons may have gone to Syria to fight, and police say they are arresting increasing numbers of people either on their way to fight or suspected of links to the conflict.

An uprising against the Syrian president Bashar al-Assad began three years ago and has become increasingly bloody with a number of jihadist groups joining the fight to bring down the regime. The UN's human rights office said last July – since when it has stopped updating its death toll – that at least 100,000 people have been killed in the conflict.

Britain's most senior terrorism prosecutor, Sue Hemming, has said that Britons who travelled to fight in Syria against Bashar al-Assad could face life sentences under terrorism offences on their return to the UK.