The United States joined Britain's allies in Europe and around the world on Monday in expelling scores of suspected Russian spies in an unprecedented response to a nerve agent attack.

At least 113 alleged agents working under diplomatic cover were ordered out by 21 governments, dwarfing similar measures in even the most notorious Cold War spying disputes, and marking a British diplomatic victory.

Washington led the way, ordering out 60 Russians, in a new blow to US-Russia ties less than a week after President Donald Trump congratulated Vladimir Putin on his re-election.

Canada, Ukraine, Albania and most European Union states matched the move with smaller-scale expulsions, after Britain urged allies to respond to the poisoning of double agent Sergei Skripal.

Russia has denied it was behind the attempted assassination, which left Skripal and his daughter gravely ill in perhaps the first nerve agent attack in Europe since World War II.

And it warned that there would be a tit-for-tat response to those countries “pandering to British authorities” without, Moscow claims, fully understanding what had happened.

But Western officials made it clear in announcing the expulsions that they share Britain's assessment that only the Kremlin could have been behind the March 4 incident in Salisbury, England.

White House spokeswoman Sarah Sanders said Washington and its allies were acting “in response to Russia's use of a military-grade chemical weapon on the soil of the United Kingdom.”

The strong language contrasted with the warm words Trump shared with Putin last week, when he overrode his advisers' concerns and congratulated his opposite number Putin on his election win.

“The United States stands ready to cooperate to build a better relationship with Russia, but this can only happen with a change in the Russian government's behavior,” Sanders said.

Consulate closed

US officials said that 48 “intelligence officers” attached to Russian diplomatic missions in the US would be expelled, along with 12 accredited to the United Nations in New York.

Trump's ambassador to the UN, Nikki Haley, welcomed the move and said: “Here in New York, Russia uses the United Nations as a safe haven for dangerous activities within our own borders.” Spokeswomen for the White House and the State Department, along with Haley and US ambassador to Moscow Jon Huntsman condemned the alleged Russian attack.

But Trump himself, who usually likes to tweet or hold a press event for major announcements, was silent.

In addition, the Russian consulate general in Seattle will be closed, the White House said, because of its proximity to a US submarine base and a plant run by private aerospace giant Boeing.

This represents the largest US expulsion of Russian or Soviet agents ever and comes after Trump's predecessor Barack Obama expelled 35 in late 2016 over alleged election meddling.

Russia's foreign ministry warned that the “unfriendly step by this group of countries will not pass without trace and we will respond to it.” And the Russian embassy in Washington appeared to hint at what this response would be.

In a tweet, the Russian mission asked followers to vote on which US consulate should be closed, listing those in Vladivostok, St Petersburg and Yekaterinberg as options.

Russia's ambassador to the United States condemned the decision and told state-run Sputnik News: “A serious blow to the quantitative and qualitative composition of the Russian embassy in Washington, DC.”

It was not immediately clear how many Russians are assigned to its various US missions, but in 2016 Putin ordered the United States to reduce its Moscow personnel to 455 to achieve parity.

Before the measures were announced, Russia's Foreign Minister Sergei Lavrov had accused Britain of “feverishly trying to force allies to take confrontational steps.” Canada confirmed it was expelling four Russians, Ukraine 13, Albania two and Norway two. At least 16 EU member states were kicking out agents.

An unofficial AFP tally brought the total number of suspects to 112. They are due to leave in the days and weeks to come.

Britain welcomed its allies' decision as a diplomatic and moral victory, after concerns that some would prefer not to offend Moscow despite international horror over the attack.