Moscow said Friday it could hit back at Britain at "any minute" with its own raft of punitive measures after the West blamed Russia directly for a nerve agent attack on a former double agent.
President Vladimir Putin's spokesman Dmitry Peskov on Friday said Russia's sanctions could come "any minute" even though he declined to say whether Moscow would deliver its response before Sunday's presidential election.
"All the steps will be well thought out," he told reporters.
In a rare joint statement, the leaders of Britain, France, Germany and the US on Thursday condemned the attack on former double agent Sergei Skripal and his daughter Yulia as an "assault on UK sovereignty".
Moscow has vehemently denied it had a hand in the poisoning of its former spy in the cathedral city of Salisbury early this month.
Britain's key allies 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.
Russia said earlier this week it would expel British diplomats in response to London's move as well as adopt other measures that the Kremlin said would "most suit Moscow's interests."
"Of course we will," Russian Foreign Minister Sergei Lavrov told journalists when asked whether Moscow would respond in kind.
He did not provide further details.
Moscow's ambassador to Britain Alexander Yakovenko said that the majority of the Russian diplomats would leave the country next week.
The crisis unravelled in the thick of a Russian presidential campaign, with Putin expected to win a fourth Kremlin term on Sunday.
- 'Fate of a traitor' -
The Russian president has barely weighed in on the row, only telling a BBC reporter earlier 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.
Many Russians remain sceptical that the state was involved in an attack on British soil and some analysts didn't rule out the involvement of ordinary criminals or rogue agents.
The official military newspaper Krasnaya Zvezda (Red Star) interviewed the former head of Russia's GRU military intelligence, Fyodor Ladygin, who denied his officers had been involved.
"We don't care about the fate of a traitor," said Colonel General Ladygin.
"For an intelligence officer, a traitor dies immediately -- he absolutely ceases to exist in the memory. For a traitor, oblivion is death."
The attack on the Skripals revived memories of the fate of Alexander Litvinenko, a Russian dissident who died of Polonium poisoning in a 2006 attack in the UK that London blamed on Moscow.
The leaders of Britain, France, Germany and the United States said Thursday there was "no plausible alternative explanation" for the use of the Soviet-designed nerve agent.
In a joint statement, they demanded Moscow "address all questions" related to the attack against Skripal, which they said amounted to a "breach of international law".
But on Friday, Britain's opposition Labour leader Jeremy Corbyn poured cold water on claims the Russian state was involved, suggesting instead that "mafia-like groups" could have been responsible.
Skripal moved to Britain 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.
The Daily Telegraph reported late Thursday that intelligence agencies now believe the nerve agent used on the pair was planted in the daughter's suitcase before she left Moscow.
- 'We don't want Cold War' -
Vil Mirzayanov, a Soviet-era chemist who helped create Novichok, denied that Skripal's poison could have come from stocks in the former USSR and said that terrorists also could not produce it.
"To create its components one needs powerful labs and very experienced personnel which only exist in several countries," he told Russian independent newspaper Novaya Gazeta.
Mirzaynov was sacked after revealing the existence of Moscow's classified programme to produce Novichok and now lives in the United States.
Tatyana Stanovaya, a Paris-based analyst with Moscow's Center of Political Technologies, said that even if Moscow was not involved the scandal played into its hands.
"This becomes a warning to all those members of security services who are dreaming of a quiet home in Salisbury," she told AFP.
May warned more measures could follow, noting that the US-led NATO alliance and the UN Security Council had discussed the attack, while it was also expected to be on the agenda at a European Union summit next week.
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, we don't want a new arms race, Russia is our neighbour therefore we have to continue to strive for an improved better relationship with Russia," he told BBC radio.