BRUSSELS, March 15 (Xinhua) -- NATO Secretary General Jens Stoltenberg on Thursday expressed the Allies' support to Britain over nerve agent attack and called on Russia to address Britain's questions.

Addressing a press conference at NATO headquarters, Stoltenberg called the nerve agent "attack" in Britain "the first offensive use of a nerve agent on Alliance territory since NATO's foundation."

"NATO Allies agreed that the attack was a clear breach of international norms," he said, adding it is "unacceptable".

Stoltenberg said he will discuss this issue with British Foreign Secretary Boris Johnson on Monday.

Britain on Wednesday briefed the North Atlantic Council, NATO's political decision-making body, the use of a nerve agent in Salisbury.

"The UK confirmed the use of a military-grade nerve agent of a type developed by Russia and briefed Allies that it was highly likely that Russia was responsible," said the statement by the Council.

On March 4, former Russian intelligence officer Sergei Skripal and his daughter were found unconscious on a bench in a shopping center in the southern England city of Salisbury. They remain in a critical but stable condition.

The Russian government, however, has denied any involvement in the attack. Russian Foreign Minister Sergei Lavrov said on Tuesday that Russia was ready to cooperate with Britain in the investigation of the incident in accordance with international law.

"Russia is innocent and Russia is ready to cooperate in accordance with the Chemical Weapons Convention if the United Kingdom condescends to honoring its international legal obligations related to the mentioned document," Lavrov told a press conference in Moscow.