NATO has warned that the nerve agent attack on Sergei Skripal and his daughter must have "consequences" and Stoltenberg will hold a news conference in Brussels at 3:30 pm (1330 GMT) to make an announcement on "further decisions", the alliance said in a brief statement.

Governments of 24 countries around the world have expelled more than 100 Russian diplomats over the poisoning of the Skripals, an unprecedented international response to the first offensive use of a nerve agent in Europe since World War II.

Britain hailed the mass expulsions as a "turning point" for the West's attitude to a "reckless" Russia, but Moscow denies responsiblity for the poisoning and has vowed to carry out tit-for-tat measures of its own.

Relations between NATO and Russia were already at a low ebb over Moscow's annexation of Crimea and its role in the Ukraine and Syria conflicts.

Russian double agent Skripal and his daughter Yulia were poisoned with a nerve agent identified by Britain as the Soviet-made "Novichok" in the English city of Salisbury on March 4.

Skripal, a Russian military intelligence officer imprisoned by Moscow for passing on information about Russian agents in various European countries, came to Britain in a 2010 spy swap.