The UK decided to expel the diplomats after Moscow refused to explain how a Russian-made nerve agent was used on a former spy in Salisbury. 

Earlier, former US Secretary of State Rex Tillerson had called it an egregious act and said that Russia must face appropriately serious consequence.

US Press Secretary Sarah Sanders said White House wants to ensure that 'this kind of abhorrent attack does not happen again'.

"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.

Later, US ambassador to the UN, Nikki Haley said that the US shares a special relationship with the UK and would 'always be there' for the latter.

Earlier, Moscow was given a night's time to explain why a Russian nerve agent was used in the poisoning of former government spy Sergei Skripal and his daughter in the United Kingdom.

Both the spy and his daughter, Yulia, is said to be critical but stable in a hopspital.

Aged 66 and 33 respectively, the father-daughter duo were found slumped on a bench in Salisbury city centre, a BBC report read.

Meanwhile, Moscow has denied the allegations and said it will take appropriate steps.

Russian ambassador to the UN, Vasily Nebenzya, demanded proof from the British government based on which it has levied the charges.

"We were given an ultimatum and requested in 24 hours to admit that we committed a crime. In other words, confess," he said. "We do not speak the language of ultimatums. We do not use that language with anyone. And we will not allow to be spoken to in that language either."

According to the new order, checks on private flights, customs and freight will be increased.

Certain Russian assets in the UK will also be freezed.

All planned high-level bilateral contacts between London and Moscow have been suspended.

Who is Sergei Skripal?

Sergei Skripal, 66, was born in Russia on 23 June 1951.

He joined the elite Soviet airborne troop known as the Desantniki and travelled to Afghanistan.

Following his stint in Afghanistan, Skripal graduated from Diplomatic Military Academy in Moscow.

Sergei Skripal. Image: Screengrab from YouTube 

Later, he was picked up GRU - Russia's military intelligence, and worked for them as a spy.

Under cover as a diplomat, he travelled to Europe and worked as a spy.

In between 1980-1990, Skripal was allegedly approached by the UK intelligence and he is said to have turned a double agent.

In early 2000, he quit the GRU.

He was arrested for spying by Russian authorities in Dec 2004 and was sentenced 13 years in a labour camp.

According to reports, Skripal, a championship boxer in his youth, was equal to the task of settling scores with goons who were allegedly placed in the camp to rough him up.

In July 2010, he was released as part of a major spy swap.

In 2012, he lost his wife Liudmila to cancer.

Five years later, he lost his son Sasha, who died in St Petersburg.

His son's death is said to be suspicious.

White House Image: Zach Rudisin/Wikipedia