Yulia Skripal, the daughter of former Russian double agent Sergei Skripal, is said to be "improving rapidly" and is no longer in a critical condition, according to the hospital in Salisbury, England, where she is being treated.
The Kremlin has accused Britain of whipping up an anti-Russia campaign and sought to cast doubt on the British analysis that Moscow was responsible.
US and other Western nations have expelled more than 150 Russian diplomats over the poisoning. Relations between Russia and Montenegro have been strained since October 2016, following an alleged Russian-backed plot to kill Montenegro's prime minister, overthrow the government and prevent the ex-Yugoslav republic joining North Atlantic Treaty Organisation.
Lavrov said Moscow will also retaliate to the US decision to shut the Russian consulate in Seattle by closing the USA consulate in St. Petersburg.
Thursday's announcement follows the expulsion of more than 150 Russian diplomats by European Union nations, the United States, NATO and other countries in response to the March 4 poisoning of Sergei Skripal and his daughter.
USA officials said the scale of the expulsions was based not only on the expansion of Russian espionage in the United States, but also on its increasing focus on critical infrastructure targets such as electrical grids, financial networks, transportation and health care.
Russia's Foreign Ministry said on Wednesday the circumstances of the Salisbury attack led Russian Federation to suspect possible involvement of the British intelligence services.
The leaders endorsed the Western response to Russia's chemical weapons attack in Britain, including the expulsion of Russian diplomatic personnel, the White House said.
The Russian embassy in Washington responded by asking its Twitter followers to vote on which U.S. consulate should be closed, listing those in Vladivostok, St Petersburg and Yekaterinburg as options.
Yulia Skripal's father Sergei remains in critical condition.
"I'm pleased to be able to report an improvement in the condition of Yulia Skripal", Christine Blanshard, Medical Director for Salisbury District Hospital, said.
Her father remained in a critical but stable condition, the hospital said.
London and its allies have blamed Moscow, citing the use of a Soviet-designed nerve agent Novichok, Russia's record of targeting dissidents and its history of aggression in recent years, from Crimea to cyber-attacks.
Since emerging from the world of high espionage and betrayal, Skripal lived modestly in the cathedral city of Salisbury and kept out of the spotlight until he was found unconscious on March 4.
The former Soviet republic of Georgia says it will expel a Russia diplomat in solidarity with Britain over the nerve agent poisoning of a former Russian spy.
Nations also expelling Russia's diplomats are Ukraine, France, Germany, Poland, Lithuania, Czech Republic, Italy, Australia, Netherlands, Spain, Albania, Denmark, Hungary, Ireland, Macedonia, Sweden, Norway, Latvia, Estonia, Finland, Romania, Croatia and Maldova. Russian diplomats have been operating out of the special interests section of the Swiss Embassy in the Georgian capital, Tbilisi, since 2009. The ministry condemned the poisoning, calling it a "serious challenge to common security".