There is a “real prospect” an unconventional attack by Russia against NATO – such as an act of sabotage or arson – will cause “substantial” casualties, a top alliance official has told Sky News.

James Appathurai, who is updating a NATO strategy to track and deter so-called hybrid warfare, said allies must be clearer among themselves and with Moscow about what level of grey zone hostilities could trigger an allied response, including the use of military force.

He said NATO’s 32 member states were already in a “boiling frog” situation, with suspected Russian hybrid attacks across Europe, the United States and Canada creeping up to a volume that would have been “utterly unacceptable” five years ago.

There had been a particular rise in more “kinetic” acts – like cutting vital undersea cables, sabotage against buildings and the planting of incendiary devices inside aircraft cargo – since Russia launched its full-scale invasion of Ukraine in February 2022.

“We can definitely count dozens. Up to 100 for sure. But then there’s a lot of foiled plots,” Mr Appathurai, NATO’s deputy assistant secretary general for innovation, hybrid and cyber, told Sky News in an interview at the alliance’s headquarters in Brussels.

He said the increase in attacks was a response by the Kremlin to Western military support to Ukraine as well as a belief that the West is anti-Russia – something the official said was not true – and is trying to constrain Moscow from attacking its neighbours. “That part’s true. So they don’t like what we’re doing, but also they see us as an enemy. And that’s getting worse.”

Russia has previously denied allegations of sabotage, cyber hacks and assassinations.

Asked if he was worried a suspected Russian hybrid attack could breach a threshold that would prompt NATO to invoke its Article 5 collective response – whereby an attack on one is deemed an attack on all – and go to war with Russia, Mr Appathurai said: “What really worries me is that one of these attacks, as I say, will break through in a big way.”

He pointed to an attempt by Russia in 2018 to kill Sergei Skripal, a former Russian double agent, and his daughter Yulia, in Salisbury using a batch of a novichok chemical weapon that contained enough poison potentially to kill thousands of people.

“So there is a real prospect of one of these attacks causing substantial numbers of casualties or very substantial economic damage,” Mr Appathurai said.

He added: “And then what we don’t want is to be in a situation where we have not thought through what we do next.

“So that’s part of the reason why we’re going to exercise all of this. And that includes military elements of the response.”

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His team is updating a NATO strategy to understand, deter and counter hybrid warfare that was last drawn up in 2015 when the threat was very different.

The work includes a new effort by the alliance to plot all suspected hybrid attacks by Russia and other hostile actors, including China, Iran and North Korea, to have a better understanding of the scale and scope of the challenge.

The updated policy – which is due to be approved at a summit in 2025 – will also set out how NATO can better deter aggression and how it should respond – given that any move by the alliance could be deemed escalatory.

“We’re in a little bit of a boiling frog situation,” Mr Appathurai said.

He continued: “We are seeing now what would have been utterly unacceptable five years ago, but we’ve kind of gotten used to it… And that’s very dangerous.

“So we want to establish a baseline now, then prevent escalation, manage it if it happens, but also work to de-escalate, not to where we are now, but to where we were years ago.”

Since its foundation in 1949, NATO allies have been deterring the then-Soviet Union and now Russia from launching conventional military attacks on its soil.

There is a clear red line – well understood by both sides – about how any kind of armed attack could trigger a collective Article 5 response.

The alliance has said hybrid hostilities – which are deliberately hard to attribute and could be carried out by criminals acting unwittingly on behalf of the Russian intelligence services – could reach the level of a hybrid attack that might require the same kind of armed response.

However, the threshold is unclear.

On whether NATO needs to be better at setting out to Russia what its red lines are when it comes to hybrid warfare, Mr Appathurai said: “What we need to do now is be clearer among ourselves and then decide how we communicate that also to the Russians, that there are no-go areas.

“So we do need and are working on being more clear about what these red bands – these areas are, these thresholds.”