The parents and brothers of a British man who was killed while fighting in Syria with Islamic State have been arrested by counter-terrorism police in Portsmouth.

Police raided the family home of Ifthekar Jaman, 23, in Southsea, Hampshire, and arrested his brothers, Tuhim, 26, and Mustakim, 23, on suspicion of being involved in a Syria-related terrorism plot.

The men’s father, Enu Miah, 57, and mother, Hena Choudhury, 48, were also arrested on suspicion of failing to disclose information about acts of terrorism.

Ifthekar Jaman, who was killed fighting with Islamic State in December last year, encouraged others to follow him and take up arms against the Syrian regime. He was killed shortly after he had given an interview to BBC Newsnight.

Jaman was the first of a group of young men from Portsmouth to travel to Syria last year. One of the men – Mushudur Choudhury – returned and was convicted of a terror offence earlier this year. Another has been killed in Syria.

Sources said since the men departed for Syria, two or three others from the city may have travelled to join them.

The four members of the Jaman family were held as part of an operation by the south-east counter-terrorism unit that also saw the arrest of a 23-year-old woman from Farnborough and a 29-year-old woman from Greenwich, south London, on suspicion of the commission, preparation and instigation of acts of terrorism.

The operation was not regarded as being as serious as last week’s arrests in London over an alleged terrorist attack plot.

Thames Valley police said Tuesday’s arrests were linked to suspected terrorist activity around “conflicts overseas” and not to any immediate threat to communities in the UK. A spokesman later confirmed that the arrests were related to the conflict in Syria.

Hampshire police said the arrests were part of a nationwide operation. “This is an issue across the country and not specific to any one region or area,” it said in a statement.

There have been 14 arrests under counter-terrorism powers in the last week. Three men were detained by counter-terrorism police in central London on Monday and five men were detained last week by detectives investigating a potential plot to attack Britain.

The bulk of the work of counter-terrorism investigators at present is the apparent threat posed by Islamic State, the militant group that has taken over large tracts of Syrian and Iraqi territory.

Police were standing guard on Tuesday outside Jaman’s home in Portsmouth. The door window was smashed and police tape surrounded the property. Officers arrived at the scene with large evidence bags.

A neighbour said the house had been raided previously by police. “The police have been here before. There were lots of them, all with their lights on and everything.”

Another neighbour, who did not want to be named, said: “I was woken up this morning by the police running in and shouting.”

Police urged people from local communities with any concerns or information to contact them on 101 or the anti-terrorist hotline 0800 789 321.