The fight against terrorism has blurred the traditional lines of war and peace, the chief of Nato has said, stressing that there is a "constant threat" of attack.
In an exclusive, wide-ranging interview with the Press Association, Secretary General Jens Stoltenberg also said it was very difficult to predict when the fight against Islamic State will be over.
His comments come following a two-day defence ministers meeting in Brussels where it was announced Nato plans to expand its military training mission in Iraq following the fall of IS.
Pressed on how vulnerable Europe is to the threat of Islamic militancy and returning fighters from Iraq and Syria, he said "significant achievements" have been made in the fight against terrorism.
"There is a constant threat of terrorist attacks, we have seen that that has happened again and again against Nato allies," he added.
"And the threat is present, that is the reason why we have to step up our efforts to fight terrorism and we need many different tools."
He said the neighbourhoods where people are recruited need to be addressed, and that social workers, schools, police, intelligence services, border controls and military means were also important.
Mr Stoltenberg highlighted how 98% of the territory once controlled by the murderous group in Iraq and Syria when they seized vast swathes of the countries, has been liberated.
"These are significant achievements in the fight against terrorism," he added, stating that due to intelligence sharing "many terrorist attacks have been prevented".
"There is still a threat, there is still a danger and this is a generational fight. It also illustrates that the concept of peace and war has changed," Mr Stoltenberg said.
Historically he said it has been easier to define periods of conflict, but added: "The fight against Daesh or Isil - it is impossible to say exactly when it started.
"It has taken place in Iraq and Syria but also in Afghanistan, North Africa and Asia, in cyber space, and I think it will be very hard to provide an exact date of when that war or that fight ends.
"The fight against terrorism illustrates that it is a much more blurred line between war, conflict and non-conflict than how it was before.
"That just makes it important that we continue the efforts."
Drawing attention to the rise of the extreme right in his home country of Norway, as well as the murder of Labour MP Jo Cox, he said it illustrates how it is "not about a fight against religion".
"This is a fight against terrorist acts, acts of violence or acts or crimes, regardless of what kind of ideology or religion they try to use as an excuse for totally unacceptable actions," he added.
His comments come amid concerns of a diplomatic row between the US and the UK over the fate of two British men suspected of being members of an IS group dubbed the Beatles.
Washington is keen for militants captured by the coalition-backed Syrian Democratic Forces to be turned over to face justice in their home countries.
But Britain's Defence Secretary Gavin Williamson said he does not want Alexanda Kotey and El Shafee Elsheikh to "ever step foot" in the UK again.
Quizzed on what he thinks should happen, Mr Stoltenberg said it is up to individual Nato allies to decide the fate of accused extremists.
"We are concerned about foreign fighters, they have been responsible for brutal violent crimes and the return of foreign fighters can pose a threat to Nato allies," he said.
"Therefore we have to deal with that in a good way.
"But it is for individual Nato allies to decide. Nato will not develop a common policy for the 29 allies, it will be up to each ally to decide - including Britain."