April 5 (UPI) -- Buses carrying expelled diplomatic staffers left the U.S. embassy in Moscow Thursday, the deadline for the American representatives to leave the country in what's been a growing response to the poisoning of a former Kremlin spy.
Dozens of U.S. Foreign Service representatives and their families left for the airport -- including 58 employees of the U.S. Embassy in Moscow and two staffers of the consulate in Russia's fourth-largest city, Yekaterinburg.
Russia expelled the diplomats last month to answer the U.S. government's removal of 60 Russian agents from diplomatic missions and the closure of the Russian consulate in Seattle.
Moscow's relationships with Western nations have rapidly deteriorated since former Russian spy Sergei Skripal and his daughter were poisoned in a nerve agent attack in Britain March 4. Britain, the United States and other nations have blamed the Kremlin for the attack. Russian officials have denied any involvement.
British Prime Minister Theresa May also expelled 23 Russian diplomats from Britain, and more than 20 other nations have said they will oust Moscow representatives in response to the poisoning.
Foreign Minister Sergey Lavrov said Western nations have persuaded other countries to follow suit.
"The Skripal case was a fictional and trumped-up pretext to unreasonably expel Russian diplomats, without producing any evidence, not only from the U.S. and the U.K. but also from many other countries, who were arm-twisted into going along," Larov said.
Thursday, British officials reiterated claims that Russia was behind the attack, which officials say involved the use of a Soviet-era nerve agent.
"That nerve agent has been identified to have been manufactured, we believe in Russia, and we believe that the nerve agent, the Novichok type of nerve agent is only capable of being produced by a nation state," British Security Minister Ben Wallace said Thursday.
"We can say that all roads lead to Russia, that we are beyond reasonable doubt that the Russian state is behind this."
Russian officials have demanded that Britain furnish proof to support claims that Moscow was behind the attack. British scientists said this week they haven't proven Moscow was the source of the nerve agent.
A Russian request to the Organization for the Prohibition of Chemical Weapons to launch a joint probe with Britain into the nerve agent attack was denied Wednesday.
"It is a diversionary tactic, and yet more disinformation designed to evade the questions the Russian authorities must answer," the British delegation to the OPCW said.
Following the denial, Russia demanded an open session of the United Nations Security Council be convened on Thursday over the case.