NATO Secretary-General Jens Stoltenberg said Britain could count on NATO's solidarity, but that it had not invoked the alliance's mutual defence clause.
"There has been no request for article 5. It's important that we react in a proportional, measured and defensive way", he told reporters at NATO headquarters in Brussels.
Stoltenberg is in uncharted waters, explains Kristine Berzina, a fellow at the Brussels office of the German Marshall Fund of the United States.
"The attack in Salisbury is very alarming to NATO at large. It shows that the conflict with the presumed attacker - and this case has been attributed to Russia - that this kind of conflict does not have to stay somewhere outside NATO territory.
This is a conflict that can come on to the territory. And the Secretary General spoke today about this being one incident in a pattern of incidents. That is very alarming to the aliance."
The poisoning of the ex-Russian spy on British soil came just days before the Russian presidential election - and that is a bizarre timing, according to the Russian military analyst Alexander Golts.
"Russia had no need to poison the former colonel of the military intelligence. Moreover the timing is extremely unfortunate. We know the tradition of the Soviet and - I believe Russian - special services not to conduct active operations before important political events.
It was possibly committed by somebody who is playing his own game. Before we understand his intention, it would be very hard to defend it."