Asked on Friday whether Moscow would expel United Kingdom diplomats, his foreign minister Sergei Lavrov said: "Of course we will".
Russia's foreign ministry said the diplomats in Moscow have one week to leave. A row with Britain over allegations the Kremlin used a nerve agent to poison a Russian double agent in a sleepy English town - denied by Moscow - has not dented his standing.
Prime Minister Theresa May announced the expulsion of 23 Russian diplomats on Wednesday in response to the poisoning case and Britain is waiting on the Kremlin's response, which is expected to include tit-for-tat expulsions.
"We have said on different levels and occasions that Russian Federation has nothing to do with this story", Kremlin spokesman Dmitry Peskov said in a text message responding to Johnson's claim.
Putin needs healthy turnout to keep potential challengers at bay and supporters happy, says Chris Weafer of economic and political consultancy Macro Advisory.
A Russian election monitoring group says government authorities forced the closure of a center created to collect complaints of violations in Sunday's presidential vote.
Polls show none of the seven candidates running against Putin is a threat, which could also put off voters.More news: The Science World Celebrates The Life Of Stephen Hawking
Greenblatt also said it was "deeply disturbing to see the Russian president giving new life to classic anti-Semitic stereotypes that have plagued his country for hundreds of years".
Yevgeny Prigozhin, who ran the St. Petersburg-based Internet Research Agency, has been targeted by the USA sanctions along with 12 of the agency's employees. Most statements have fallen somewhere in between the two extremes.
British communications regulator Ofcom said it could review the licence of the Kremlin-backed RT broadcaster if Russian involvement in the poisoning were proven.
The Skripals were found slumped over on a park bench in the cathedral town of Salisbury near the famed ruin of Stonehenge.
Skripal moved to the United Kingdom in a 2010 spy swap and had taken his daughter, who was on a visit from Moscow, out for lunch before they both collapsed on a bench in the street on March 4.
"In his campaign rally in Crimea today, President Putin reiterated Russia's false claims to Ukrainian territory in another open admission that the Russian government disdains the global order and disrespects the territorial integrity of sovereign nations", Nauert said.
Putin's recent refusal to extradite them, the Democrats said, was "simply unacceptable" and the US should challenge that through diplomatic channels or an global arrest warrant.