"The UK is a highly valued ally, and this incident is of great concern to NATO. NATO is in touch with the UK authorities on this issue," Stoltenberg said in a statement issued by his office.

Stoltenberg made his remark hours after British Prime Minister Theresa May said it was "highly likely" that Russia was responsible for the poisoning of one of its former spies, an accusation rejected by Moscow.

In a dramatic escalation of diplomatic tensions over the attack on Sergei Skripal and his daughter on March 4, May said the nerve agent that poisoned them was a military-grade type developed by Russia.

"The use of any nerve agent is horrendous and completely unacceptable," Stoltenberg said.

The strong words by the NATO chief will increase speculation that Britain will consider triggering "article five" of NATO membership, a rule that says an attack on one member constitutes an attack on all.

But according to the BBC, May's government has declined to turn to the highly exceptional clause and will seek other ways to confront Russia.

May said Britain had demanded a full explanation from the Russian government by the end of Tuesday.

Without a "credible response", May said Britain will conclude it was "an unlawful use of force by the Russian State against the UK", and pledged to outline a "full range of measures" in response.