The leaders of the United States, Germany and France have joined together with the United Kingdom in accusing Russia of being behind the nerve agent attack on an ex-Russian spy.

In a joint statement Thursday, President Trump, French President Emmanuel Macron, German Chancellor Angela Merkel and British Prime Minister Theresa May said they "abhor" the attack against Sergei Skripal, 66, and his 33-year-old daughter, Yulia, in Salisbury on March 4. A police officer who came to the pair's aid was also sickened.

"This use of a military-grade nerve agent, of a type developed by Russia, constitutes the first offensive use of a nerve agent in Europe since the Second World War," the statement said.

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“It is an assault on U.K. sovereignty and any such use by a State party is a clear violation of the Chemical Weapons Convention and a breach of international law. It threatens the security of us all,” they said.

The quartet stated “there is no plausible alternative explanation” to Russian responsibility for the attack on British soil and the Kremlin’s failure to respond to a “legitimate request” for an explanation “further underlines its responsibility.”

“We call on Russia to live up to its responsibilities as a member of the UN Security Council to uphold international peace and security,” the statement ended.

The leaders’ remarks come a day after Britain expelled 23 Russian diplomats and suspended high-level contacts with Moscow over the incident.

Later on Thursday, Trump told reporters that “it looks like” Russia was behind the attack.

“I spoke with the prime minister and we are in deep discussions in a very sad situation,” he said. “It certainly looks like the Russians were behind it. It’s something that should never, ever happen and we are taking it very seriously.”

Russia is expected to take retaliatory measures soon.

Kremlin spokesman Dmitry Peskov wouldn't reveal how Russia will retaliate, but said the decision will come from President Vladimir Putin, "and there is no doubt that he will choose the option that best reflects Russian interests."

"We are worried by this situation" and will work patiently to express Russia's position on the international stage, Peskov said.

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British authorities have said Skripal and his daughter were poisoned with a military-grade nerve agent called Novichok, which was developed by the former Soviet Union during the end of the Cold War. It is believed to be 10 times more potent than other nerve agents such as VX and sarin and his highly undetectable.

On Tuesday, May said Russian officials were treating the incident with “sarcasm, contempt and defiance.”

“[Russia] has shown complete disdain for the gravity of these events,” she told lawmakers, adding the Kremlin's alleged actions represent “an unlawful use of force against the United Kingdom.”

Russia has denied any involvement in the poisoning of Skripal and his daughter, who remain hospitalized and in critical condition. A police officer was also exposed to the nerve agent and was hospitalized.

The Associated Press contributed to this report.