EU ministers, without explicitly blaming Russia or threatening any repercussions, called on Moscow to "address urgently" British questions over its Novichok nerve agent programme.
The European Union on Monday condemned the poisoning in Britain of a former Russian spy, and British Foreign Secretary Boris Johnson described Moscow's claims that it was not responsible for the attack as absurd.
"The lives of many citizens were threatened by this reckless and illegal act," EU foreign ministers said in a statement in Brussels, where they were briefed by Johnson on the latest developments surrounding the March 4 poisoning in Salisbury that left Sergei Skripal and his daughter in critical condition.
Without explicitly blaming Russia or threatening any repercussions, the ministers called on Russia to "address urgently" British questions over its Novichok nerve agent programme, and they expressed "unqualified solidarity" with Prime Minister Theresa May's government.
TRT World's Sarah Morice explains what's happening.
Johnson told reporters that "Russian denials grow increasingly absurd," with contradictory claims about whether Russia produced Novichok used in the attack.
"What people can see is that this is a classic Russian strategy of trying to conceal the needle of truth in a haystack of lies and obfuscation," he said.
Johnson said many EU countries have been victims of "malign Russian behavior" and that Moscow was "not fooling anybody anymore."
May and EU leaders are set to discuss the attack at a summit in Brussels on Thursday night.
The statement came as international chemical weapons experts were due on Monday to take samples of the nerve agent for independent testing.
Russian President Vladimir Putin on Sunday dismissed the British accusations as "nonsense," but said that Moscow was ready to co-operate with London in the investigation.
In his first comments on the incident, Putin referred to the poisoning of the ex-Russian spy and his daughter as a "tragedy," but said that if the British claim that they were poisoned by the Soviet-designed nerve agent were true, the victims would have been killed instantly.
But German Foreign Minister Heiko Maas said the EU stood beside Britain, and its analysis of what happened.
"We have been informed in great detail in recent days by our colleagues in Britain. All the information we have suggests that there is no alternative plausible explanation to the Russian side bearing a share of the responsibility." Maas said.
French Foreign Minister Jean-Yves Le Drian agreed, saying: "We share the British analysis."
Swedish Foreign Minister Margot Wallstrom rejected Russian suggestions that her country might have stockpiles of Novichok.
"This is just ridiculous and totally unfounded. I think that they are trying to make some kind of diversion from the real issues here." she said.
Sweden said Monday it had summoned Russia's ambassador for talks after Moscow suggested the Scandinavian country may have produced a deadly nerve agent to poison a former Russian spy in Britain.
Foreign ministry spokesman Per Enerud confirmed media reports that the ambassador had been summoned to a meeting to take place at the ministry on Tuesday, prompted by the allegation.
"Yes that is correct," he told AFP.
The Kremlin made the accusation after denying allegations by London and allies that it was behind the March 4 attack on Sergei Skripal and his daughter Yulia in the English city of Salisbury.
"The most likely source of this chemical attack are the countries that, since the end of the 1990s -- and currently still -- have been carrying out intense research on the substances from the 'Novichok' programme," Russian foreign ministry spokeswoman Maria Zakharova said Saturday.
"These countries are Britain, Slovakia, the Czech Republic, Sweden. The question should also be raised regarding the United States."