Four world leaders have issued a joint statement blaming Russia for the poisoning of Sergei Skripal, the former double agent who fell ill with his daughter in England.

The leaders say there is "no plausible alternative explanation" to Russia being to blame.

The statement follows days of diplomacy by UK Prime Minister Theresa May and comes amid a meeting by NATO to discuss the crisis.

LONDON — The leaders of the United States, Britain, France, and Germany have released a joint statement condemning Russia for the poisoning of Sergei Skripal in England last week.

US President Donald Trump, UK Prime Minister Theresa May, French President Emmanuel Macron, and German Chancellor Angela Merkel agreed that there was "no plausible alternative explanation" than that Russia was to blame for the attack.

The leaders described the poisoning as "the first offensive use of a nerve agent in Europe since the Second World War," adding that Russia's actions "threaten the security of us all."

The statement marks a significant ramping up of tensions with Russia and comes as members of the NATO military alliance meet to discuss the crisis.

It came after several days of diplomacy by May following initial reluctance by the French and US governments to publicly blame Russia for the incident, in which Skripal and his daughter were poisoned by what authorities identified as a Russia-developed nerve agent.

On Wednesday, May announced that Britain would expel 23 Russian diplomats from the UK in response to the attack.

Britain is also set to freeze the accounts of people close to Russian President Vladimir Putin and suspend all high-level contact with the Russian government.

On Thursday, UK Defence Secretary Gavin Williamson said Russia should "shut up" and "go away" and suggested that Britain was in a "chilly" war with the country.

The joint statement in full

"We, the leaders of France, Germany, the United States and the United Kingdom, abhor the attack that took place against Sergei and Yulia Skripal in Salisbury, UK, on 4 March 2018. A British police officer who was also exposed in the attack remains seriously ill, and the lives of many innocent British citizens have been threatened. We express our sympathies to them all, and our admiration for the UK police and emergency services for their courageous response.

"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. It is an assault on UK 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. The United Kingdom briefed thoroughly its allies that it was highly likely that Russia was responsible for the attack. We share the UK assessment that there is no plausible alternative explanation, and note that Russia's failure to address the legitimate request by the UK government further underlines its responsibility.

"We call on Russia to address all questions related to the attack in Salisbury. Russia should in particular provide full and complete disclosure of the Novichok programme to the Organisation for the Prohibition of Chemical Weapons (OPCW). Our concerns are also heightened against the background of a pattern of earlier irresponsible Russian behaviour. We call on Russia to live up to its responsibilities as a member of the UN Security Council to uphold international peace and security."

Asked about the poisoning on Thursday, Trump told reporters that it "certainly looks like Russia were behind it."

Donald Trump says "it looks like" Russia was behind the Salisbury attack pic.twitter.com/TLyZp273R9 — Muhemmed Asfand Yar (@muhemmedasfand) March 15, 2018

A Russian foreign ministry spokeswoman hit back on Thursday describing attempts to blame Russia for the poisoning as "insane."

Maria Zakharova said she wanted to comment on "the statements made by the prime minister of Britain in parliament with completely insane accusations towards the Russian Federation, our entire country, our entire people."

She added that Russia planned to retaliate soon, saying they would "introduce reciprocal steps, without doubt... They are currently being worked on and will be adopted in the near future."