Expelled Russian diplomats set to leave US Saturday: ambassador
The American flag flies outside the US consulate in Saint Petersburg, which will be closed in response to an escalating row over an attack on former spy AFP/File / OLGA MALTSEVA

Sixty Russian diplomats expelled by the United States in response to an apparent attempt to assassinate a former double agent in Britain with a nerve agent were set to return home on Saturday, Moscow's envoy said.

"On March 31 all the diplomats who were declared persona non grata are flying home together with their families," Russia's ambassador to the US, Anatoly Antonov, told reporters.

In total, 171 people will leave the country, state news agency TASS reported Antonov as saying.

The Russian government provided two planes for the evacuation and one of them will make a brief stopover in New York to collect 14 families, he added.

Washington this week joined a score of Britain's allies in expelling Russian diplomats in response to the attack, with a total of more than 150 now ordered out of the US, EU members, NATO countries and other nations.

The US said it was expelling 60 alleged Russian "spies" posted around the country and at the Russian mission to the United Nations.

Russia's consulate in Seattle will also be closed, but the total size of Russia's diplomatic footprint in the US might not shrink for long, because the expelled staff could be replaced, the US authorities have said.

Moscow responded on Thursday by announcing that it was expelling 60 US diplomats and closing Washington's consulate in Saint Petersburg.

On Friday, Russia expelled even more diplomats from 23 countries, most of them European Union member states.

The tit-for-tat measures were retaliation to the coordinated expulsion of Russian diplomats by Britain and its allies over a nerve agent attack against former double agent Sergei Skripal and his daughter Yulia in the English city of Salisbury on March 4.

Britain has said it is "highly likely" that Russia was responsible for the Skripal attack using the Novichok nerve agent developed in the Soviet Union, but Russia has angrily denied any involvement.