LONDON - British police requested military assistance in investigating a nerve agent attack on a former Russian double agent as speculation mounted Friday about how London could respond if a state actor were to blame.

Police extended the cordon around the modest suburban home of Sergei Skripal in Salisbury, the quiet city in southwestern England where he and his daughter Yulia were found slumped on a bench on Sunday.

The pair remain unconscious in a critical but stable condition, while Nick Bailey, one of the first police officers on the scene, is now sitting up and talking after initially being admitted to intensive care.

With police also hurt in the attack, pressure is intensifying on PM Theresa May to find and punish the culprits.

The involvement of "a British citizen, especially a policeman, requires the immediate and strong involvement of the British authorities," Chatham House analyst Mathieu Boulegue told AFP.

Around 21 people have been treated, according to Kier Pritchard, chief constable for Wiltshire Police.

"A number of those have been through the hospital treatment process, they're having blood tests, they're having treatment in terms of support and advice," he said. Authorities are racing to find the source of the nerve agent used against 66-year-old Skripal, who came to Britain in 2010 as part of a spy swap, as politicians warned it bore the hallmarks of an attack by Russia. National counter-terrorism police, who are leading the investigation, announced on Friday that they had requested assistance from the military "to remove a number of vehicles and objects from the scene".

"Military assistance will continue as necessary during this investigation," they added, with media reporting 108 personnel may be involved, including Royal Marines and chemical speicliasts.

Defence Secretary Gavin Williamson said the armed forces "have the right people with the right skills" to deal with the "crucial inquiry".

During a visit to the site on Friday, interior minister Amber Rudd called the attack "outrageous" but urged people to "give the police the space they need to really go through the area carefully, to do their investigation". In response to questions over Russia's possible involvement, May has said that "if action needs to be taken then the government will do that".

Possible responses include the expulsion of some of Russia's 58 diplomats, some kind of boycott of the 2018 football World Cup or an increased British military presence in eastern Europe.

Foreign Secretary Boris Johnson floated the idea of deepening sanctions against Russian officials, but Britain would have to persuade its international partners.

Moscow has reacted angrily to the accusations it was involved, with Foreign Minister Sergei Lavrov on Friday dismissing them as baseless "propaganda".

On Wednesday, however, Russian state TV presenter Kirill Kleimyonov had appeared to deliver a veiled warning, saying: "Don't choose England as your future country of residence... whether you are a professional traitor to the motherland or whether you hate your country in your spare time."

- A deadly gift? -

Analyst Boulegue said it would be "extremely difficult" to pinpoint who committed the attack, and that the response could be a lot more complicated if it turned out that internal factions were responsible.

"It is possible that it is related to the elections in Russia or that it is part of a battle between the elites within the security services, to send messages either to the English or to the Russian elites," he said.

Police have cordoned off the bench where the pair were found, as well as an Italian restaurant and a pub they visited before their collapse.

The also sealed off the grave of Skripal's wife, Liudmila, who died in 2012 from cancer, as well as the memorial stone of his son, Alexander, who was cremated last year after reportedly dying of liver problems.

The Times newspaper said police were probing whether Skripal's daughter, who arrived in Britain from Moscow last week, may have inadvertently brought in the nerve agent as a gift.

Skripal was a former colonel in Russian military intelligence who was jailed in his country for betraying agents to Britain's MI6 secret service.

He was pardoned in 2010 before being flown to Britain as part of a high-profile spy swap involving Russia and the United States.