London, Mar 6 (IANS): The Kremlin on Tuesday said it has "no information" about what could have led to a former Colonel in Russian military intelligence -- convicted of spying for Britain -- collapsing in Salisbury.

But Moscow said it was willing to co-operate in the police investigation, the Russian media reported.

The British police were trying to identify what substance rendered Sergei Skripal, 66 -- who was granted refuge in the UK in 2010 under a "spy swap" -- and a 33-year-old woman critically ill in hospital.

The pair were were found unconscious on a bench at a shopping centre on Sunday.

Col. Skripal was jailed for 13 years by Russia in 2006. He was convicted of passing the identities of Russian intelligence agents working undercover in Europe to the UK's Secret Intelligence Service, MI6.

Russian President Vladimir Putin's spokesman Dmitry Peskov told journalists that "Moscow was prepared to help with the investigation", Tass news agency reported.

"We see this tragic situation but we don't have information on what could have led to this, what he was engaged in," he said.

Wiltshire Police said the pair, found at The Maltings shopping centre in Salisbury, had no visible injuries -- but still they were investigating whether a crime had been committed.

Meanwhile, the police closed the nearby Zizzi restaurant "as a precaution" following the incident, the BBC reported.

Temporary Assistant Chief Constable Craig Holden said: "They are currently being treated for suspected exposure to an unknown substance.

"The focus is on trying to establish what has caused these people to become critically ill."

He said the police's "major incident" response was not a counter-terrorism investigation, but that multiple agencies were involved and police were keeping an "open mind".