The joint statement by the 29 member countries of the US-led alliance said the attack against Sergei Skripal and his daughter Yulia in Salisbury, southwest England, was a "clear breach of international norms and agreements".

"Allies expressed deep concern at the first offensive use of a nerve agent on Alliance territory since NATO's foundation," said the statement by the North Atlantic Council, which groups NATO allies.

"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", they added.

British Prime Minister Theresa May has said a "military grade" nerve agent known as Novichok was used in the March 4 incident.

"Allies agreed that the attack was a clear breach of international norms and agreements," the statement said.

NATO Secretary General Jens Stoltenberg added in a tweet: "I welcome that the 29 NATO Allies have just agreed a statement expressing full solidarity with the UK and offering support after the Salisbury attack."

The attack has raised speculation that Britain will consider triggering "article five" of NATO membership, a rule that says an attack on one member constitutes an attack on all.

Britain has received broad international support in the wake of the poisoning, which also left a policeman seriously ill.

EU President Donald Tusk said earlier Wednesday that Russia was "most likely" behind the attack and that leaders of the 28 European Union states were likely to discuss the issue at a Brussels summit next week.

The UN Security Council is set to hold an emergency meeting on the poisoning later Wednesday at Britain's request.