The leaders of Britain, France, Germany and the U.S. have jointly condemned the poisoning of a former Russian spy on British soil that has been blamed on Moscow, calling it "the first offensive use of a nerve agent in Europe since the Second World War."
A joint statement issued Thursday by Prime Minister Theresa May, President Emmanual Macron, Chancellor Angela Merkel and President Donald Trump said there was no plausible alternative explanation to Russia's involvement in the attack. "We call on Russia to address all questions related to the attack," the leaders said.
Later at the White House, President Trump said "it certainly looks like the Russians" were responsible.
WATCH: Trump on Russia spy poisoning
Russia has denied any involvement.
Former spy Sergei Skripal and his daughter, Yulia, were found unconscious on a park bench in the English town of Salisbury and rushed to the hospital, where they remain in serious condition. Several other individuals, including a police officer, were sickened.
British Prime Minister May announced a series of reprisals against Russia over the poisoning, including the expulsion of 23 Russian diplomats. It will be the largest expulsion of Russian diplomats since 1971, at the height of the Cold War.
Russian Foreign Minister Sergey Lavrov said Moscow will expel British diplomats "soon" in retaliation. In remarks Thursday in Moscow, Lavrov called May's actions "boorish" and said it was intended to distract from Britain's difficult negotiations in leaving the European Union.
In Brussels, NATO Secretary General Jens Stoltenberg called for a "proportionate" British response to a nerve agent attack, saying the incident must have "consequences".
The White House issued a statement Wednesday that said the U.S. "stands in solidarity with its closest ally, the United Kingdom" and shares in the assessment "that Russia is responsible for the reckless nerve agent attack on a British citizen and his daughter, and we support Britain's decision to expel Russian diplomats as a just response.
British officials said the chemical nerve agent known as Novichok was used in the attack. It was developed by the Soviet Union and inherited by Russia.
Skripal, a former agent of Russia's military intelligence agency, was arrested in 2004 for betraying dozens of Russian agents to Britain. He was freed in 2010 as part of an exchange of spies with the United States, and eventually settled in Britain.
French President Emmanuel Macron said Thursday that his government will take "measures" in the coming days in response to the poisoning of Skripal and his daughter.
Prime Minister May traveled to Salisbury later Thursday to speak with residents and officials.