Tokyo - Russia's foreign minister threatened on Wednesday to retaliate against Britain for "anti-Russian measures", with the two countries at loggerheads over the poisoning of a spy in southern England.
Speaking after a meeting with Japanese counterpart Taro Kono, Sergei Lavrov said: "If the British government continues taking some anti-Russian measures, we will hit back under the principle of reciprocity."
Lavrov urged the British government to "respond calmly" over the March 4 attack on former spy Sergei Skripal and his daughter Yulia, who remain in critical condition.
"Overall, there is no doubt that the British leadership has knowingly chosen to undermine the British-Russian relationship," said the minister.
Britain says only Russia had the capability, motive and intent to launch the attack, which used the nerve agent Novichok reportedly developed by the former Soviet Union.
Russian President Vladimir Putin has dismissed allegations of Moscow's responsibility as "nonsense".
Britain reacted by expelling 23 Russian diplomats and their families - around 80 people in total - and has also cut off high-level contacts.
A spokesperson for Prime Minister Theresa May said London was "actively considering" other measures.
On Tuesday, the head of the OPCW chemical watchdog said it would take two to three weeks to complete laboratory analysis of samples taken from the poisoning.
The affair has further damaged Russia's already shaky relations with many Western countries.
The EU has expressed its solidarity with Britain and leaders at a summit later this week will agree to "coordinate on the consequences" for Russia, according to a draft statement seen by AFP.
US Defence Secretary Jim Mattis suggested on Tuesday that Moscow's suspected involvement shows Russia has "chosen to be a strategic competitor".
However, President Donald Trump skipped the issue when congratulating Putin on his re-election and proposed a summit in the "not-too-distant future".
Japanese Prime Minister Shinzo Abe voiced "outrage" over the attack in a call to May, according to her office.
Skripal, 66, a former Russian officer who sold secrets to Britain and moved there in a 2010 spy swap, remains in a coma along with his 33-year-old daughter after they were found unconscious on a bench in Salisbury.