NATO allies face the “constant threat of terrorist attacks”, the organisation’s secretary general has said.

Jens Stoltenberg said the traditional lines of war and peace had become blurred, as he spoke at the conclusion of a two-day meeting of defence ministers yesterday.

Attended by UK Defence Secretary Gavin Williamson, the Brussels summit included the announcement of plans to expand military training in Iraq following the fall of Daesh in the country.

When questioned on the threat posed to Europe by Islamist extremism, Stoltenberg said: “There is a constant threat of terrorist attacks, we have seen that that has happened again and again against Nato allies.

“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.”

Stoltenberg highlighted the achievements of those battling the group in Iraq and Syria, where 98 per cent of its territory has been liberated. He said: “These are significant achievements in the fight against terrorism.

“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.

“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 cyberspace, 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.”

However, drawing attention to the rise of the extreme right in Norway, as well as the murder of Labour MP Jo Cox, he went on: “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.”

The comments come amid concerns of a diplomatic row between the US and the UK over the fate of two UK citizens suspected of being members of a Daesh 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 Williamson does not want Alexanda Kotey and El Shafee Elsheikh to “ever step foot” in the UK again.

Stoltenberg said: “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.

“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.”

Commenting on the role of the international alliance, Williamson said: “Nato is the cornerstone of our defence but we live in an uncertain world, with the confrontation and conflict we face shifting and evolving.

“Nato must modernise at the same pace, so we can respond better and faster to deter those that threaten our safety and way of life.”

Stoltenberg’s comments on cyber attacks come after the UK blamed Russia for a hit on Ukraine last year that spread across Europe. The NotPetya attack focused on Ukraine’s financial, administrative and energy sectors, but was engineered to spread further.

Williamson said: “Russia is ripping up the rule book by undermining democracy, wrecking livelihoods by targeting critical infrastructure and weaponising information. We must be primed and ready to tackle these stark and intensifying threats.”