US sanctions against Russia tied to a nerve agent attack in Britain, which were announced earlier this month, will officially take effect on Monday, according to a notice posted at the Federal Register.

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.