The United States says it is imposing new sanctions on Russia over Moscow's involvement in the use of a "lethal" nerve agent in the attempted killing of a former spy in Britain.

Britain holds an emergency cabinet meeting over a couple who were left critically ill after being exposed to the same nerve agent used on a former Russian spy earlier this year.

The world's chemical arms watchdog says it had confirmed Britain's findings on a nerve agent used in an attack on a former spy last month, which London said came from Russia.

Yulia Skripal has been discharged from hospital more than a month after she was poisoned by a military-grade nerve agent together with her father, a former Russian spy, the English hospital treating them says.

Russia tells Britain at the United Nations Security Council that "you're playing with fire and you'll be sorry" over its accusations that Moscow was to blame for poisoning a former Russian spy and his daughter.

Yulia Skripal, who was poisoned with a nerve agent in an English city last month along with her Russian ex-spy father Sergei, says she is recovering, according to a police statement issued on her behalf.

Moscow tells Britain it must cut just over 50 more of its diplomatic and technical staff in Russia as a standoff deepened over the poisoning of a Russian former spy and his daughter in England, the Russian Foreign Ministry says.

The daughter of former Russian spy Sergei Skripal, who with her father was poisoned with a nerve agent in Britain earlier this month, is no longer in a critical condition and is improving rapidly, the hospital treating the pair says..