Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE on Tuesday called the poisoning of a former Russian spy on British soil "attempted murder," saying Russian President Vladimir Putin should be "responsible as the head of state."

Mattis was asked by reporters at the Pentagon how he would qualify Moscow’s actions surrounding the recent poisoning of Sergei Skripal and his daughter, Yulia, with a military-grade nerve agent in Salisbury, England.

“Attempted murder of a man and his daughter, how’s that for starters?” Mattis replied.

Mattis said the "pretty obvious" Russian use of a chemical agent was deployed "for the first time in Europe since World War II."

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NATO earlier in the day said it was expelling seven diplomats from Russia's mission to the alliance and denying accreditation to three more as part of a coordinated effort with other countries to punish Moscow for the poisoning.

President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE on Monday moved to expel 60 Russian officials from the U.S. as part of the response, and 24 other countries are also expelling Russians. The United Kingdom already kicked out 23 Russian diplomats earlier this month.

Moscow denies the allegation that it conducted the attack.

When asked whether he thought Putin was aware of the effort to poison Skripal and his daughter, Mattis said he wouldn’t speculate on what the Russia president knew, but added “certainly he’s responsible as the head of state.”

Mattis also said that Russia "has the potential to be a partner with Europe," but instead has "chosen to seek a different relationship with the NATO nations."

"They’re doing things that they believe are deniable," Mattis said.

"They take the insignia off soldiers’ uniforms and they go into Crimea. ... They point out that they can’t be proven that they tried to kill the person in Salisbury. ... They’re trying to break the unity of the Western alliance, NATO."