NATO's chief says the use of a nerve agent on a former Russian spy is a threat to international security and allies have offered "practical support" to Britain if it requests help.

Secretary-General Jens Stoltenberg said Thursday that the poisoning of Sergei Skripal and his daughter in England on March 4, blamed on Russia, "is unacceptable. It has no place in a civilized world," The Tribune reported.

Stoltenberg told reporters that the attack is "a blatant and serious violation of norms and rules that are important for the security of all of us."

He said the attack seems to be part of a "reckless pattern of Russian behavior over many years."

Stoltenberg said he will discuss the attack next week with British Foreign Secretary Boris Johnson.