NATO Secretary-General Jens Stoltenberg addresses a news conference during a NATO defence ministers meeting at the Alliance headquarters in Brussels, Belgium, February 15, 2018. REUTERS/Francois Lenoir

BRUSSELS (Reuters) - The Western NATO military alliance called on Russia on Wednesday to give Britain “complete disclosure” of the Soviet-era nerve agent used in an attack on a Russian double agent on March 4, following a British briefing to allies at NATO headquarters.

“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.

“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 programme to the Organisation for the Prohibition of Chemical Weapons,” the statement said, referring to the watchdog in The Hague.