LONDON (AFP) - Former Russian spy Sergei Skripal and his daughter first came into contact with a nerve agent at their home address in Britain, police said on Wednesday (March 28) as the high-profile probe into the attack continues.

"At this point in our investigation, we believe the Skripals first came into contact with the nerve agent from their front door," said Dean Haydon from London's Metropolitan Police.

The March 4 attack on the Skripals in the English city of Salisbury has been met with a major response that has seen more than 150 Russian diplomats expelled from countries around the world.

British authorities have blamed Moscow, which denies any involvement, and said a Soviet-designed nerve agent dubbed Novichok was used in the poisoning.

"Traces of the nerve agent have been found at some of the other scenes detectives have been working at over the past few weeks, but at lower concentrations to that found at the home address," said Haydon.

Investigators had earlier sealed off the bench where the Skripals were found, a pub and restaurant the pair visited, and the grave of the former spy's wife.

Police said around 250 counter-terrorism detectives are currently working on the case, which could continue for months.

"Those living in the Skripals' neighbourhood can expect to see officers carrying out searches as part of this but I want to reassure them that the risk remains low and our searches are precautionary," said Haydon.

The Skripals remain in a coma in hospital, while a police officer who responded to the attack was discharged last week.