By Agence France-Presse
The war of words between Moscow and London over the nerve agent attack on a former Russian spy escalated as Britain's Foreign Secretary Boris Johnson said his government's "quarrel" was with Putin rather than the Russian people.
"We think it overwhelmingly likely that it was his decision to direct the use of a nerve agent on the streets of the UK, on the streets of Europe, for the first time since the Second World War," Johnson said in London.
Putin's spokesman Dmitry Peskov responded saying Johnson's claims violated all rules of diplomatic protocol.
Linking Putin to the attack on Sergei Skripal, who moved to Britain in a 2010 spy swap, "is nothing but shocking and unforgivable behaviour from the point of view of diplomacy", Peskov told Russian news agencies.
The crisis has unravelled in the thick of Russia's presidential campaign, with Putin expected to win a fourth Kremlin term on Sunday.
In a rare joint statement, the leaders of Britain, France, Germany and the United States on Thursday condemned the attack on Skripal and his daughter Yulia -- both in a critical but stable condition in hospital -- as an "assault on UK sovereignty".
They said there "no plausible alternative explanation" for the use of the Soviet-designed nerve agent other than Russian responsibility.
Police officer Nick Bailey, who initially attended to the Skripals, was also in hospital in a critical state, though conscious, but his health has improved and he is now in a stable condition, England's health service said Friday.
Alexander Yakovenko, Russia's ambassador to Britain, told Channel 4 television that Britain's response to the attack was a "gross provocation".
He branded the British investigation "untransparent and secret", adding that there was "no proof" that Skripal was gravely ill.
Moscow opens probes
The Kremlin has vehemently denied it had a hand in the poisoning of its former spy in the English city of Salisbury on March 4.
London's key allies have closed ranks against Putin after British Prime Minister Theresa May announced the expulsion of 23 Russian diplomats and suspended high-level contacts, among other measures.
On Friday, Russia said it could hit back at Britain at "any minute" with its own raft of punitive measures.
The Kremlin has indicated it would expel British diplomats in a riposte to London's move as well as adopt other measures that would "most suit Moscow's interests".
"All the steps will be well thought out," Peskov said.
Meanwhile the Investigative Committee, which reports to Putin, opened a probe into the "attempted premeditated murder" of Skripal's daughter, a Russian national, which it said had been "carried out in a way that was dangerous to the public".
At the same time a separate probe was opened into the mysterious death of Nikolai Glushkov, a Russian exile who was found dead at his London home this week.
London's Metropolitan Police later announced their own murder investigation, saying Glushkov had suffered compression of the neck.
- Scepticism in Russia -
Putin has barely weighed in on the row, telling a BBC reporter this week: "Sort things out from your side and then we will discuss this with you."
Russia insists it had no motive to target Skripal with what Britain says was a highly potent Soviet-designed nerve agent called Novichok, in the first such attack in Europe since World War II.
Skripal had taken his daughter, who was on a visit from Moscow, out for lunch before they both collapsed on a bench.
Many Russians remain sceptical that the state was responsible and some analysts have not ruled out the involvement of ordinary criminals or rogue agents.
The incident revived memories of the fate of Alexander Litvinenko, a Russian dissident who died of Polonium radiation poisoning in a 2006 attack in Britain that London blamed on Moscow.
EU president Donald Tusk said Friday that the bloc would discuss the attack at a Brussels summit next week, adding it would send a "clear message".
- 'We don't want Cold War' -
Vil Mirzayanov, a Soviet-era chemist who helped create Novichok but later revealed the existence of Moscow's classified programme, said terrorists could not produce it.
"To create its components one needs powerful labs and very experienced personnel which only exist in several countries," the now US-based whistleblower told opposition newspaper Novaya Gazeta.
Britain on Friday said that it had invited the chemical weapons watchdog OPCW to take a sample of the poison for analysis, under Article 8 of the Chemical Weapons Convention.
May warned more measures could follow against Russia, noting that the US-led NATO alliance and the UN Security Council had discussed the attack.
NATO Secretary General Jens Stoltenberg said Friday the alliance did not want a return to Cold War hostilities with Russia while expressing support for Britain's stance.
"We don't want a new Cold War," he told BBC radio.