Canada, along with France, Germany and the United States backed Britain and pledged on Thursday to work to disrupt "the hostile activities of foreign intelligence networks" and called on Russia to disclose its nerve agent programme.

Britain's security minister says Russian President Vladimir Putin bears ultimate responsibility for the Novichok nerve agent attack carried out in England.

British prosecutors said Wednesday they have charged two Russian men with the nerve agent poisoning of ex-spy Sergei Skripal and his daughter Yulia in the English city of Salisbury.

Russia condemned a new round of U.S. sanctions as draconian on Thursday after news of the measures sent the rouble tumbling to two-year lows and sparked a wider asset sell-off over fears that Moscow was locked in a spiral of never-ending sanctions.

The State Department said the U.S. this week made the determination that Russia had used the Novichok nerve agent to poison Sergei Skripal and his daughter, Yulia, and that sanctions would follow.

"The fact that a nerve agent was used to do this is shocking. I don't want to describe the details but the clinical treatment was invasive, painful and depressing," Yulia Skripal said.

Russia wants Britain to provide information about the pets that lived in the house of the Russian former spy who was poisoned with nerve agent in England.

The daughter of a Russian ex-spy poisoned in a nerve-agent attack is out of critical condition — an unexpected piece of good news in a chilling mystery that has sparked an international crisis.

Experts say the recent mass exodus of diplomats is largely intended make clear to Putin's administration that actions such as these will not be tolerated by the West.  

Stoltenberg says, despite the expulsions, Russia will still have a diplomatic mission of 20 people at alliance headquarters in Brussels and that will allow Russia to keep essential contacts with NATO members.