British Prime Minister Theresa May is expected to announce measures against Russia on March 14, after Moscow ignored a midnight deadline to explain how a nerve agent developed during the Cold War was used to poison a former Russian spy in Britain.
The United States, European Union, and NATO all said they supported Britain after May said it was "highly likely" that Russia was behind the poisoning of Sergei Skripal and his daughter with a deadly chemical substance developed by the Soviet military -- part of a group of nerve agents known as Novichok.
May gave Moscow a deadline of midnight on March 13 to explain how the rare nerve agent made its way to England. She said she is reviewing a range of economic and diplomatic measures in retaliation for the assault.
The prime minister is due to host a meeting of her national security council. She is then expected to give an update to lawmakers.
Russia has denied involvement and said it would not respond to May's ultimatum until it receives samples of the suspected toxin.
British police, meanwhile, said the probe into the March 4 poisoning of the Skripals will last many weeks and that they are not ready to identify any persons of interest in the inquiry. The two remained in critical condition in a hospital in the southern English city of Salisbury.
The White House joined Britain in calling on Russia to provide "unambiguous answers regarding how this chemical weapon, developed in Russia, came to be used in the United Kingdom," after a phone call between May and U.S. President Donald Trump on March 13.
"The two leaders agreed on the need for consequences for those who use these heinous weapons in flagrant violation of international norms," it said.
French President Emmanuel Macron and German Chancellor Angela Merkel also condemned the attack and offered support to Britain.
Both before and after the deadline had passed, Russia's Embassy in London said Moscow will not respond to London’s ultimatum until it receives samples of the chemical substance.
"Any threat to take ‘punitive’ measures against Russia will meet with a response. The British side should be aware of that," it said on Twitter.
Russian Foreign Minister Sergei Lavrov told reporters in Moscow that his country's requests to see samples of the nerve agent have been turned down. He denied Moscow was behind the poisoning, saying that "Russia is not guilty."
Lavrov called the accusations "nonsense" and said, "We have nothing to do with this."
A Russian chemist who helped develop the nerve agent in the Soviet era and now lives in the United States said that only the Russian government could have carried out the attack with such a deadly and advanced toxin.
Vil Mirzayanov, 83, said that Russia maintains tight control over its Novichok stockpile and that the agent is too complicated for a nonstate actor to have weaponized.
“The Kremlin all the time, like all criminals, denying -- it doesn’t mean anything,” Reuters quoted Mirzayanov as saying in an interview at his home in Princeton, New Jersey.
Britain's ambassador to the Organization for the Prohibition of Chemical Weapons (OPCW) said that Russia had failed to declare its full stockpile of nerve agents to the global oversight body.
Peter Wilson told reporters that Russia, which President Vladimir Putin said in 2017 had destroyed all its chemical weapons under OPCW supervision, has in fact "failed for many years" to fully disclose its chemical weapons program.
Wilson repeated assertions by the British government that Russia was "implicated" in the attack and demanded that Moscow now declare its undisclosed program.
Also on March 13, the chief of the OPCW said it is "extremely worrying that chemical agents are still being used to harm people."
"Those found responsible for this use must be held accountable for their actions," OPCW Director-General Ahmet Uzumcu said in a speech to the group's executive council.
Uzumcu said that British Foreign Secretary Boris Johnson called him on March 12 to inform him of the results of investigations into the attack on Skripal, 66, and his 33-year-old daughter Yulia.
Meanwhile, British broadcasting regulator Ofcom has warned it could review the license of Kremlin-backed broadcaster RT.
Ofcom said it had written to ANO TV Novosti, which holds RT's British broadcast licenses, saying that if Moscow is found to be behind the attack, "we would consider this relevant to our ongoing duty to be satisfied that RT is fit and proper."
In response to Ofcom's statement, Moscow threatened on March 13 to bar all British media from working in Russia if British authorities ban RT.
"No British media will work in Russia if they close down RT," Russian Foreign Ministry spokeswoman Maria Zakharova said.
Skripal, a retired Russian military intelligence colonel, was convicted by a Moscow military court in 2006 of "high treason" for passing secrets to Britain's Secret Intelligence Service.
He was one of four Russian prisoners released in 2010 in exchange for 10 Russian sleeper agents detained in the United States in one of the biggest spy scandals since the Cold War.
Police have confirmed that Sergei Skripal is a British citizen. Lavrov said that Yulia Skripal has Russian citizenship.
With reporting by AP, AFP, dpa, and Reuters