Ambassador Jon Hunstman was summoned to the Russian Foreign Ministry, a gothic skyscraper built under Soviet dictator Josef Stalin, and told that 60 diplomats from U.S. missions had a week to leave Russia, as Washington had expelled 60 Russians.
A statement by Scotland Yard said specialists have identified the highest concentration so far of the nerve agent as being "on the front door of the address".
Trump reiterated his long-standing call for better relations with Moscow, despite a series of conflagrations and crises, including Russian meddling in the USA election and the poisoning of a Russian double agent on British soil.
Porton Down chief executive Gary Aitkenhead said Tuesday that scientists at the lab "have not verified the precise source, but we provided the scientific information to the government who have then used a number of other sources to piece together the conclusions that they have come to". Russian officials are seeking to visit her in hospital.
The State Department says that a number of Russian diplomats expelled from a Russian consulate in Washington state could be replaced by other Russian officials, noting that the same is true for a number of American officials recently expelled from a consulate in St. Petersberg.
Russian Federation denies responsibility and has suggested the poison may have come from Britain. The Foreign Office has said there was "not an ounce of truth" in his implication the nerve agent could have been linked to Porton Down.More news: Australia sevens captain in hospital after head punch
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And when asked to clarify what he meant, he added: "A real war".
Russian foreign minister Sergey Lavrov appeared to suggested on Monday that United Kingdom secret services may have been involved in the March 4th attack in Salisbury, which he said may have been "beneficial" to the British government to distract attention from Brexit.
Britain blames Russian Federation for the pair's poisoning with a Soviet-developed nerve agent.
A top Russian diplomat says the nerve agent poisoning of a former Russian spy may have been arranged by Britain to justify military spending.
The US ambassador was also ordered to shut the consulate in St Petersburg, in Russia's retaliation for the biggest expulsion of diplomats since the Cold War.
Buzhinsky told the BBC the expelling of diplomats by both countries over the poisoning of Sergei Skripal and his daughter Yulia in Salisbury could lead to war. British authorities suspect Skripal was poisoned by a Soviet-made nerve agent.
On March 26, in the wake of the Skripal incident, a number of European Union member countries, the United States, Canada and Australia announced the expulsion of Russian diplomats.