Three more people were being questioned last night after the discovery of Semtex, detonators, handguns and ammunition in west Belfast.
A 45-year-old man was arrested in Sunderland in north-east England, while a 21-year-old woman and a man, aged 29, were held in west Belfast.
The find, which included half a kilo of Semtex, came during a police operation at Ballymurphy Road that began on Thursday night and lasted into Friday morning.
The weapons and explosives were discovered as part of an investigation into "violent dissident terrorist activity".
A 67-year-old man and a 38-year-old woman arrested at the time were released at the weekend pending a report to the Public Prosecution Service.
The arrest in England yesterday morning followed a joint operation by detectives from the PSNI Serious Crime Branch and the North East Counter Terrorism Unit with officers from Northumbria Police.
Northumbria Police told residents in Sunderland there is “nothing to suggest that there was, or is, any risk to people in the area”.
Chief Superintendent Andy McDyer said: "We have extra officers in the area to reassure people.”
Unionists have raised concerns that the explosives could have originated in Provisional IRA dumps.
DUP MP Jeffrey Donaldson said his party will be seeking a meeting with police to seek answers, while Ulster Unionist MP Tom Elliott also expressed alarm.
Deputy First Minister Martin McGuinness praised the PSNI for the seizure but described the Semtex discovery as a “worrying development”.
He said it “shows what many people have accepted for a very long time, that there were people within mainstream Irish republicanism, who left and who, in leaving, took some equipment with them. I think it’s as simple as that”.
