NATO Secretary-general Jens Stoltenberg | Dan Kitwood/Getty Images NATO ‘in touch’ with UK about nerve gas attack Jens Stoltenberg calls attack on ex-Russian spy a ‘great concern.’

NATO Secretary-General Jens Stoltenberg said Monday the alliance is in contact with the U.K. about the nerve agent attack on a former Russian double agent, amid speculation that the alliance could invoke its collective defense clause if Russia is found responsible.

British Prime Minister Theresa May said earlier Monday that it was "highly likely" Moscow was behind the attack on the former spy, Sergei Skripal, and his daughter in Salisbury, England because U.K. investigators had determined the military-grade chemical used was manufactured in Russia.

“The United Kingdom has concluded that Sergey 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,” he added. “The U.K. 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.”

The White House called the attack “reckless” on Monday but did not point blame at Russia.

Article 5 of the North Atlantic Treaty states that the allies “agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all,” and if such an attack occurs, they will take action as necessary “including the use of armed force.”

The decision to invoke Article 5 must be made by the North Atlantic Council, the alliance’s chief political body. The collective action to be taken would likely depend heavily on the U.K.’s response, and any request it makes for assistance. NATO has invoked Article 5 just once — after the September 11, 2001 terrorist attacks on the United States.

Russia is already facing economic sanctions by the EU and other nations, including the U.S., as punishment for Moscow’s 2014 annexation of Crimea and subsequent military intervention in eastern Ukraine.