Media playback is unsupported on your device Media caption Theresa May announces measures to send a "clear message" to Russia over the use of a nerve agent in Salisbury

The White House says it "stands in solidarity" with "its closest ally" the UK and supports its decision to expel 23 Russian diplomats.

PM Theresa May said the diplomats would be expelled after Moscow refused to explain how a Russian-made nerve agent was used on a former spy in the UK.

US President Donald Trump's spokeswoman accused Russia of undermining the security of countries worldwide.

Moscow denies responsibility and says it will expel UK diplomats "soon".

Foreign Minister Sergei Lavrov told Russian media the expulsions would "definitely" happen and reportedly referred to the UK's allegations as "absolutely boorish".

Former spy Sergei Skripal, 66, and his daughter, Yulia Skripal, 33, remain critically ill in hospital after being found slumped on a bench in Salisbury, Wiltshire, on 4 March.

'Just response'

Foreign Secretary Boris Johnson said the attack was Russia's "way of saying to people this is what happens to people who stand up to our regime".

He said: "Now is the moment for Putin to jam the lid down and send a signal to people: 'You do this, you're going to die.'"

BBC North American editor Jon Sopel said the White House statement was notable in the unqualified support it offered Theresa May.

President Trump's way of talking about Russia, using language that had not been heard from the White House before, was also significant, he said.

Image copyright EPA/ Yulia Skripal/Facebook Image caption Sergei Skripal, 66, and his daughter Yulia, 33, are in a "critical but stable" condition, Boris Johnson said

In the statement, Mr Trump's press secretary Sarah Sanders said the US wanted to ensure "this kind of abhorrent attack does not happen again" and called the UK's expulsion of Russian diplomats "a just response".

"This latest action by Russia fits into a pattern of behaviour in which Russia disregards the international rules-based order, undermines the sovereignty and security of countries worldwide, and attempts to subvert and discredit Western democratic institutions and processes," she said.

The chemical used in the attack on Mr Skripal and his daughter has been identified part of a group of nerve agents developed by Russia known as Novichok, Mrs May said.

The PM said there was "no alternative conclusion" than to believe Russia was "culpable" for the poisonings.

Mr Johnson told the BBC a sample of the nerve agent would be sent to the Organisation for the Prohibition of Chemical Weapons - the independent international body set up to stop chemical warfare - for analysis.

But he added that Russia's "smug, sarcastic" response indicated their "fundamental guilt".

Media playback is unsupported on your device Media caption The foreign secretary told Radio 4 Today the Russia tensions aren't a return to the Cold War

Moscow wanted to "simultaneously deny it and at the same time to glory in it", he said, adding the nerve agent was chosen "to show that it is Russia".

Mrs May spoke to French President Emmanuel Macron on Thursday, after a French government spokesman said the time for decisions would come "once the elements are proven".

Shortly afterwards, his office released a statement saying France agreed with the UK's claim that Russia was behind the attack.

Labour leader Jeremy Corbyn, who has condemned the "appalling attack", has faced criticism - some from his own party - after his spokesman said there was not yet definitive proof the Russian state carried out the poisoning.

Moscow had refused to meet the UK's Tuesday evening deadline to explain the use of the nerve agent, with Foreign Minister Sergei Lavrov saying it would co-operate if the UK submitted a formal request under the Chemical Weapons Convention, which gives a 10-day time limit to respond.

But when the UK's deadline passed, Mrs May announce the expulsion of 23 diplomats - the largest number it has expelled since 1985. They have been given one week to leave the country.

Other measures include increased checks on private flights, customs and freight as well as a boycott of this year's Fifa World Cup in Russia by ministers and the Royal Family.

Media playback is unsupported on your device Media caption The BBC's Paul Adams looks at why the UK is expelling 23 Russian diplomats

The BBC's Laura Kuenssberg said the UK government was "all too aware" of the likelihood of retaliation from Moscow alongside denials and scorn in the coming days.

Meanwhile, any crackdown on Russia could be painful for UK business, including BP's 20% stake in a Russian oil and gas firm, BBC business editor Simon Jack said.

The White House statement echoed earlier comments made by the US ambassador to the UN, Nikki Haley, who cited the "special relationship" between the two countries and said the US would "always be there" for the UK.

Media playback is unsupported on your device Media caption The UK, US and Russia all addressed the UN over the use of a nerve agent on UK soil

Also addressing the UN Security Council, Britain's deputy UN ambassador, Jonathan Allen, accused Russia of breaking its obligations under the Convention on the Prohibition of Chemical Weapons.

The Russian ambassador to the UN, Vasily Nebenzya, denied Moscow's involvement and demanded "material proof".