Dutch security forces have thwarted four planned terror attacks in the Netherlands since late 2011, the country's intelligence agency said Thursday.

The country's top spy chief, Rob Bertholee, told the NOS broadcaster late Wednesday that he could say his service had recently foiled "two to three" attacks, but then agreed that since he took up his post in December 2011 the force had averted three to four terror-related plots.

A spokesman for the intelligence and security agency AIVD told AFP it was four plots that had been uncovered in the past few years.

"It is of course an issue of definition," added spokesman Hilbert Bredemeijer.

Bertholee told NOS he could "dare to say that we have prevented an attack," mentioning the case of a Dutch citizen jailed for four years in November for preparing a terror strike.

The man was arrested in 2016, after police discovered an assault rifle and a large amount of fireworks in his Rotterdam apartment.

Asked by NOS how many plots had been detected, Bertholee said "recently there have been two or three which I cannot divulge because some of them are still being dealt with and are still before the courts".

He said sometimes it was only "with hindsight" that it was possible to say whether an arrest had been terror-related.

The Netherlands has so far been spared from the assaults by Islamic jihadists which have rocked its neighbours in recent years, including France, Belgium and Britain.

But amid a number of scares, and reports that people linked to some of those attacks may have crossed briefly into the country, Dutch security and intelligence officials have been on high alert.

"The main thing which is complicated here is that we are obviously investigating something which may happen," Bertholee added in the extract of his interview.

The full interview will be broadcast on NOS on January 21.

On Christmas Eve, Dutch police arrested four men including a Swedish citizen on suspicion of being involved in terror-related activities.

In another scare, Dutch military police shot and wounded a man armed with a knife at Amsterdam's Schiphol airport causing panic among travellers in mid-December. But the authorities said the man was "confused" and the incident was not terror-related.