Thirteen Ukrainian diplomats should also leave Russian Federation.
"What we are seeing here is the worsening of diplomatic relations, a tit-for-tat response", added Challands, describing the crisis as "severe".
North Atlantic Treaty Organisation also announced seven officials working in the Russian mission to the alliance will have to leave.
White House spokeswoman Sarah Huckabee Sanders says those actions were "an appropriate response" to the poisoning on British soil.
White House Press Secretary Sarah Sanders said Russia's expulsion of United States diplomats marked a "further deterioration" in relations.
Sedwill was speaking to reporters on a visit to Washington where he's consulting with US officials.
The Kremlin insisted it was not Russian Federation that had started the diplomatic war with the West.
Sedwill cited the downing of the 2014 downing of an airliner over eastern Ukraine, cyberattacks, subversion and use of mercenaries as other examples of Russian aggression.
More than 20 countries have expelled Russian envoys in solidarity with the UK.
The Foreign Ministry said it told the British ambassador on Friday that Moscow is ready to cooperate in the investigation.
The warning, posted Saturday on the embassy website, said it reflected "the anti-Russian policy, the growing threatening rhetoric of the British side (and) the British government's selective actions against Russian individuals and legal entities". She calls Russia's actions "regrettable" and "unwarranted".
"Russia should have found different mechanisms to respond without such radical measures", Zykov said.
Officials in Moscow accused London and Washington of pressuring other nations to sign up to an global campaign of "Russophobia" which, they said, could drag the world into a new Cold War. On Saturday, its London embassy issued a statement warning Russians travelling to the United Kingdom that they could be subject to "provocations" including having various items planted in their luggage. The U.S. move was in response to the poisoning of a Russian ex-spy and his daughter in an English city, which British authorities blame on Russia.
Russian Federation says it has sent a diplomatic note demanding an explanation of the search. The government's consideration will include "the rights and wishes of Yulia Skripal", it said.
"This is certainly not a surprise", Dutch Foreign Minister Stef Blok said through a spokeswoman, referring to Moscow's expulsion of two of the country's diplomats.
The Russian Foreign Ministry said the USA special services have engaged in "frantic efforts" to make cooperation offers to the expelled Russian diplomats.
Kremlin insisted it was not Russian Federation that had started the diplomatic war with the West.
-This item corrects spelling of Russian foreign minister's surname to Lavrov, not Llavrov.
"Russia did not unleash any diplomatic war", Putin's spokesman Dmitry Peskov said. He told reporters Thursday he is "very concerned".
The Russian foreign ministry said it had summoned the heads of missions from 23 countries - nearly all of them European Union member states - to tell them that some of their diplomats had to leave.
Russian Federation said Thursday it will expel an equal number of diplomats from those nations.
Russian Federation also summoned 23 foreign ambassadors to inform them that some of their diplomats would be expelled, a day after ordering 60 US diplomats to leave and demanding that Washington's consulate in St. Petersburg close on short notice.
Lavrov said the United States ambassador had been informed of "retaliatory measures", saying that "they include the expulsion of the equivalent number of diplomats and our decision to withdraw permission for the functioning of the USA consulate general in Saint Petersburg".
Lavrov said the same approach will be applied to other nations that expelled Russian diplomats this week.
Two dozen countries, including the USA, many European Union nations and North Atlantic Treaty Organisation, have ordered more than 150 Russian diplomats out this week in a show of solidarity with Britain - a massive action unseen even at the height of the Cold War. Ministry spokeswoman Maria Zakharova said there was no explanation given for the search, which she called "the latest provocation".
Russian Federation has accused Britain of failing to back up its accusations with evidence and refusing to share materials from the probe. Britain and its allies have dismissed previous Moscow claims that they possessed that type of nerve agent.
Britain says he and his daughter, who was visiting from Russian Federation, were poisoned with a nerve agent developed in Soviet times and that must have come from Russian Federation.
Yulia Skripal's father Sergei remains in critical condition.
Yulia is no longer in a critical condition and is improving rapidly, doctors said on Thursday. "Her condition is now stable".
"She has responded well to treatment but continues to receive expert clinical care 24 hours a day", said Blanshard.
Britain has said it is "highly likely" that Russian Federation was responsible for the attack using the Novichok nerve agent developed in the Soviet Union, but Russian Federation has angrily denied any involvement.
Moscow denies involvement in the attack on the Skripals.
The Trump administration had earlier ordered the expulsion of 60 Russian diplomats and shutter the countrys consulate general in Seattle.
Georgia severed diplomatic ties with Russian Federation following a brief war in the breakaway republic of South Ossetia.
A Czech official says the Czech Republic has extradited a Russian man to the United States to face charges of hacking computers at LinkedIn, Dropbox and other American companies. The ministry condemned the poisoning, calling it a "serious challenge to common security".