It was the first time a high-level official has clearly specified that Putin may have ordered the poisoning, which police are treating as attempted murder.
"Any reference or mention of our president in this regard is a shocking and unforgivable breach of diplomatic rules of decent behaviour", Kremlin spokesman Dmitry Peskov said, according to the agency. Russian Federation has asked to be allowed to examine the evidence and claims that so far it has not been allowed to do so.
The diplomatic tension increased further Friday afternoon when London's Metropolitan Police said it is treating as murder the death of Nikolai Glushkov, a close associate of Putin opponent Boris Berezovsky _ a onetime billionaire who was himself found hanging dead in 2013 in his house outside London.
A United Kingdom inquiry found that two Russian agents poisoned Litvinenko at a London hotel bar in 2006 by spiking his tea with highly radioactive polonium-210, and that Putin "probably approved" Litvinenko's killing.
British Ambassador Laurie Bristow is expected at the Russian Foreign Ministry late Saturday morning.
Russian Foreign Minister Sergey Lavrov said Moscow would "certainly" expel some British diplomats soon in retaliation.
The UK Foreign Office said its priority was looking after its staff in Russian Federation and helping those who will return to Britain.
Britain says it has invited Russian Federation to cooperate in the investigation and that this invitation hasn't been taken up.
This move came after the standoff between Britain and the Kremlin after 66-year-old Skriplal and 33-years-old daughter, Yulia, were attacked with a nerve agent in Salisbury. Peskov reiterated Russian denials of involvement in the attack that has left both Skripals in critical condition. Having failed to get a quick reply from Russia, British Prime Minister Theresa May instituted a slew of measures in response to the attack, including expelling 23 Russian diplomats (Russia has promised to retaliate), and freezing its government assets considered potentially harmful to Britons.
The risk of additional USA sanctions has increased after the Russian treasury published a list in January of oligarchs considered close to the Kremlin, as well as of top officials including Titov.
Anger over the incident has added to persistent tensions between Russian Federation and the West over a host of issues, including Russia's aggression in Ukraine and its alleged interference in elections in the United States and other Western countries.
Putin continues to campaign ahead of Sunday's elections, which he is expected to win.
"We condemn this unprecedented attack by Russian Federation on the territory of the United Kingdom".
The US treasury department said the use of a military-grade nerve agent in the Salisbury incident "further demonstrates the reckless and irresponsible conduct of its (Russia's) government".
He also has appealed to national pride, pointing at Western sanctions against Russian Federation as a proof of efforts by the US and its allies to isolate and weaken Russian Federation.
In a letter to the Times, Salisbury NHS Trust emergency medical consultant Stephen Davies said only three people - the Skripals and Det Sgt Nick Bailey - had needed treatment.
Russia's foreign minister said Britain's defense minister "lacks education".
The March 14 statement made by British Prime Minister Theresa May in Parliament on measures to "punish" Russian Federation, under the false pretext of its alleged involvement in the poisoning of Sergey Skripal and his daughter, constitutes an unprecedented, flagrant provocation that undermines the foundations of normal dialogue between our countries.