The Kremlin on Friday said it could respond to Britain's expulsion of Russian diplomats over the poisoning of a double agent "at any minute".
Retaliation would be "well thought out" and could come at any time, Kremlin spokesman Dmitry Peskov told journalists when asked if Moscow would respond to the measures before a presidential election on Sunday.
"Sooner or later the British side will have to present some kind of tangible evidence. Sooner or later we will have to move into the area of supported accusations, and not just insinuations," he added.
"We do not see that yet."
In a rare joint statement, the leaders of Britain, France, Germany and the US on Thursday condemned the nerve agent attack on Sergei Skripal and his daughter Yulia as an "assault on UK sovereignty".
As the international scandal gathers pace, Skripal and his daughter remain in critical condition after exposure to the Soviet-designed chemical Novichok on March 4 in the southwestern English city of Salisbury.
Earlier on Friday, Foreign Minister Sergei Lavrov said Moscow would "of course" respond in kind to the expulsion of 23 Russian diplomats which was announced by British Prime Minister Theresa May.
He had previously said Russia had no motive to target Skripal, but suggested others could use the poisoning to "complicate" the World Cup.
Peskov on Thursday said Russian President Vladimir Putin would ultimately choose the option that "most suits Moscow's interests".