SALISBURY (England) • Britain deployed specialist troops yesterday to help remove potential contaminants from the small English city where a Russian double agent and his daughter were poisoned.

Mr Sergei Skripal, 66, who once passed Russian secrets to Britain, and his daughter Yulia, 33, have been in intensive care since they were found slumped unconscious on a bench on Sunday afternoon in the cathedral city of Salisbury.

British Interior Minister Amber Rudd, who visited Salisbury yesterday, said they were still in serious condition, five days after collapsing.

About 180 troops including some with chemical expertise had been sent to the city to remove ambulances and other vehicles involved in the incident as well as other objects, Britain's Ministry of Defence and the police said.

"The public should not be alarmed," counter-terrorism police said in a statement.

Health chiefs have said there is a low risk to the wider public from the nerve agent used against Mr Skripal and his daughter.

The police said the duo were deliberately targeted with the rare toxin, adding that experts have identified the substance, which will help determine the source, but did not name it publicly.

Britain has said it will respond robustly if evidence shows Russia was behind the Salisbury poisoning. The Kremlin has denied any involvement in the incident and says anti-Russian hysteria is being whipped up by the British media.

"In terms of further options, that will have to wait until we're absolutely clear what the consequences could be, and what the actual source of this nerve agent has been," Ms Rudd said after visiting Salisbury and seeing the area around the bench where Mr Skripal was found, now covered by a forensics tent.

Responding to previous comments by the Interior Minister, Russia's embassy in London tweeted on Thursday: "Totally agree with Secretary @AmberRuddHR: first evidence then conclusions on Mr Skripal's case. Responsible political approach."

Twenty-one people were taken to hospital after the incident but apart from the Skripals only Detective Sergeant Nick Bailey, the first police officer on the scene, is still being treated. He remains in a serious condition although he can now talk, Ms Rudd said. She declined to give details of the investigation.

"We have to give the police all the space they need in order to collect all the information, to secure and to be able to be absolutely clear that there is no further risk," she said.

The police have cordoned off Mr Skripal's home in Salisbury, about 130km from London, and erected forensic tents in the garden.

Officers were guarding the area where he and his daughter were found, along with a pizzeria and a pub they had visited, and the graves of Mr Skripal's wife and son.

Mr Skripal betrayed dozens of Russian agents to British intelligence before his arrest in Moscow in 2004. He was sentenced to 13 years in prison in 2006, and in 2010 was given refuge in Britain after being exchanged for Russian spies.

REUTERS