Three men and a woman arrested by Dutch police as part of an investigation into an Irish criminal gang are to appear before a judge in Rotterdam later.
More than 150 guns, including automatic Glock pistols and Steyr submachine guns with silencers, were found during a raid on an Amsterdam office building.
Earlier this week, two men were arrested in Belfast and Dublin by police investigating a drugs network.
Guns were recovered in Northern Ireland and the Republic in raids on Tuesday.
Heroin worth about £3.25m was also seized by Garda officers.
ITEMS FOUND BY DUTCH POLICE
The National Prosecutor's Office in the Netherlands said on Thursday that three men aged 41, 42, and 53, and a 27-year-old woman were arrested on suspicion of possession and sale of illegal firearms.
On Friday the suspects will be brought before the investigating judge in Rotterdam.
The operation, involving the PSNI, Gardai and Dutch authorites, was launched last July following a request from the PSNI.
Sir Hugh Orde, chief constable of the PSNI, described the operation as the "most significant" his force had been involved in in six years.
Dutch police carried out four raids on Tuesday night, including three in houses - two in Amsterdam and one in the nearby city of Zaandam.
In a statement the National Prosecutor's Office said: "There were reasons to suspect that firearms and hard drugs were being supplied to the criminal organisation in Northern Ireland and that these were coming from the Netherlands."
Officers found dozens of weapons in plastic carrier bags in the boot of a car at business premises in the Oud-West area of Amsterdam.
Seven thousand rounds of ammunition were also seized, along with 20,000 euros and a currency-counting machine.
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