MOSCOW (AP) -- The Latest on the poisoning of a Russian ex-spy and his daughter in Britain (all times local):
Nearly two dozen Russian diplomats expelled by Britain over the poisoning of an ex-spy have arrived home.
Russian state television has reported that a plane carrying the diplomats and their families has landed at Moscow's Vnukovo airport.
Britain blamed Russia for the poisoning of ex-Russian spy Sergei Skripal and his daughter with a military-grade nerve agent and ordered the expulsion of 23 Russian diplomats. Moscow has responded in kind, ordering 23 British diplomats to leave. They are expected to fly home in the coming days.
Russia has fiercely denied any involvement, saying that it had destroyed its chemical weapons and had no motive to kill Skripal, who was convicted of spying for Britain but released in a 2010 spy swap.
Skripal and his daughter remain in critical condition.
The head of the Organization for the Prohibition of Chemical Weapons says it will take at least another three weeks to analyze the chemical agent used to poison a Russian ex-spy in Britain.
Ahmet Uzumcu told reporters at U.N. headquarters that OPCW experts are in the U.K. to collect samples at the British government's request.
He said once the samples are brought back to OPCW's headquarters in the Netherlands, "they will be split in our laboratory in the Hague and sent to designated labs, which may take some time."
Uzumcu added: "So you should think another three weeks ahead at least."
Britain says Soviet-designed Novichok was used to poison Sergei Skripal and his daughter in the English city of Salisbury earlier this month and has said Russia is likely responsible, which Moscow denies. They remain in critical condition.
Britain says it won't impose further sanctions on Russia over the poisoning of a former spy for now, but is keeping possible new measures under active consideration.
Prime Minister Theresa May's office says the National Security Council met Tuesday to discuss the poisoning of Sergei Skripal and his daughter Yulia.
Britain has already expelled 23 Russian diplomats and broken off high-level contacts with Moscow. Russia is kicking out 23 British diplomats in a tit-for-tat response.
Downing St. says that in addition to the expulsions, Britain has tightened checks on private flights coming into the U.K. and is preparing legislation to target the assets of human rights abusers. And it says the government is "actively considering" further measures.
Britain says the Skripals were poisoned with a Russian-made nerve agent, in violation of an international ban on chemical weapons. Russia denies responsibility.
A Russian scientist who helped create the nerve agent that allegedly was used to poison an ex-spy in England says British accusations blaming Russia for the attack are false.
Leonid Rink says the agent dubbed Novichok in the West had a different name when it was designed as a chemical weapon in the Soviet Union.
Rink told Russia's state RIA Novosti news agency Tuesday that Britain and other western nations easily could have synthesized the nerve agent after chemical expert Vil Mirzayanov emigrated to the United States and revealed the formula.
Echoing Russian government statements, Rink says it wouldn't make sense for Moscow to poison Sergei Skripal, a military intelligence officer who spied for Britain, because he was a used asset "drained" by both Russia and Britain.
He claims Britain's use of the name Novichok for the nerve agent is intended to convince the public that Russia is to blame.
Britain's National Security Council is meeting to consider more possible punitive measures against Russia over the poisoning of an ex-spy in England.
The meeting on Tuesday was happening after nearly two dozen Russian diplomats were ordered to leave Britain because of the nerve agent attack on Sergei Skripal and his adult daughter.
Russia denies involvement in the pair's poisoning. Western powers have described it as a sign of increasingly aggressive Russian meddling abroad.
British Prime Minister Theresa May and other European Union leaders are due to discuss the poisoning at a summit on Thursday.
The EU on Monday condemned the poisoning and called on Russia to "address urgently" British questions over the Soviet-developed nerve agent Novichok program.
British police investigators say it may take "months" to complete the widening inquiry. The focus is on the movement of the Skripals in the hours before they were found unconscious.
Moscow is awaiting nearly two dozen Russian diplomats ordered to leave Britain as part of a standoff over a nerve agent attack on British soil.
Britain ordered the 23 diplomats to leave by Tuesday, and they're expected in Moscow later, according to Russian media reports.
Russia retaliated by expelling 23 British diplomats, who are expected to leave Moscow in the coming days.
Russia denies involvement in the poisoning of ex-Russian spy Sergei Skripal and his daughter, Yulia, in the British city of Salisbury earlier this month. They remain in critical condition.
Britain accuses Russia of the poisoning, which Western powers see as an example of increasingly aggressive Russian meddling abroad.
International chemical weapons experts took samples Monday of the nerve agent used, which Britain says is the Soviet-developed Novichok.