"The United Kingdom has concluded that Sergei Skripal and his daughter were poisoned with a military-grade nerve agent of a type developed by Russia. And Prime Minister Theresa May stated today that it is highly likely that Russia was responsible for the act," Stoltenberg said in a statement.
"The use of any nerve agent is horrendous and completely unacceptable. 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."
Article 5 of NATO's charter, the collective security tenet, stipulates that "an armed attack against one or more [NATO member states] shall be considered an attack against them all."
Stoltenberg did not comment on Article 5.
On Tuesday, Sergei and Yulia were found unconscious on a park bench in Salisbury, the town Sergei lived in. They were rushed to the hospital, where it was revealed that they had been exposed to an "unknown substance." Sergei and Yulia are comatose and in critical condition at the time of this writing.
Scotland Yard reported that the substance was a military-grade nerve agent called Novichok, developed in the Soviet Union. "It is now clear that Mr. Skripal and his daughter were poisoned with a military-grade nerve agent of a type developed by Russia," May told the House of Commons on Sunday.
"The government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal." May added that she had summoned the Russian ambassador and demanded an explanation from Moscow. "Either this was a direct act by the Russian state against our country or the Russian government lost control of its… nerve agent and allowed it to get into the hands of others."
"Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom and I will come back to this House and set out the full range of measures we will take in response," May told the House of Commons.
US Secretary of State Rex Tillerson echoed May's assessment Monday evening. He said that the poisoning "clearly came from Russia" and that it "will trigger a response." He did not specify what sort of response.
The Russian government has denied responsibility for the incident and offered to cooperate with the British investigation. "Unfortunately, I cannot give you any comments because we do not have any information," said Kremlin spokesman Dmitry Peskov. "But, in fact, these accusations'were not long in coming,' as we say in Russia."
Dmitry Kiselyov, the host of news analysis show Vesti Nedeli on the state-owned Russia-1 channel, speculated that the Novichok may have also come from the British Ministry of Defense's chemical weapons research laboratory in Porton Down, about six miles away from Salisbury.
Skripal was a Russian military colonel and intelligence agent who changed sides to spy on the Russian military for British intelligence agency MI6. He was exposed by Russian authorities in 2004 and sentenced to 13 years in prison in 2006. Skripal was released as part of a spy exchange between Russia and the US in 2010, whereupon he moved to Salisbury.