"Yesterday was a significant moment in our response to this reckless act of aggression, but there is still more to be done as we work with worldwide partners on a long-term response to the challenge posed by Russian Federation", her spokesman said, reporting remarks made by May at a cabinet meeting.
Russian Foreign Minister Sergei Lavrov earlier in the day claimed the United States had used "colossal blackmail" to force European and other western powers to expel Russian diplomats, and promised Moscow would respond harshly to the expulsion of over 100 diplomats worldwide.
Washington's expulsions represented the largest ever of Russian or Soviet agents and came after US President Donald Trump's predecessor Barack Obama expelled 35 in late 2016 over alleged election meddling.
"This decision represents the price and the consequences of Russia's behavior", Stoltenberg said.
Nato's intervention came as it was revealed that 26 countries have now joined the United Kingdom in removing Russian envoys as part of joint action against the Kremlin.
A removal van was seen outside the Russian embassy in London on Tuesday, as Moscow is awaiting almost two dozen Russian diplomats ordered to leave Britain as part of a standoff over a nerve agent attack on British soil.
Asked on Monday about a report that the USA would also expel Russian diplomats, presidential spokesperson Dmitry Peskov said that Russia will act according to the principle of reciprocity.
In addition, the Russian consulate general in Seattle will be closed, the White House said. Moscow has promised retaliation for the diplomatic expulsions.
"Russia has the potential to be a partner with Europe ..."
This was the largest USA expulsion of Russian or Soviet agents ever and comes after Trump's predecessor Barack Obama expelled 35 in late 2016 over alleged election meddling. "It is important that we work in solidarity with each other, free people in democracies to stop Russian Federation and condemn Russian Federation for this kind of belligerent activity", Ryan said. He made no comment on the Skripal affair.