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"Allies expressed deep concern at the first offensive use of a nerve agent on Alliance territory since NATO's foundation," the alliance, founded in 1949, said in a statement, as reported by Reuters.

"Allies expressed solidarity with the UK, offered their support in the conduct of the ongoing investigation, and called on Russia to address the UK's questions including providing full and complete disclosure of the Novichok program to the Organisation for the Prohibition of Chemical Weapons," the statement said, referring to the watchdog in The Hague.