Russian opposition leader said to be suffering severe stomach pain, rapid weight loss; ambulance was called last week, but authorities refused to allow him to go to hospital

Biden administration says the measures are the first of several steps to 'respond to a number of destabilizing actions'; doesn't identify the targeted Russians

Russian president says US intelligence pushing allegations that Kremlin exposed opposition chief Alexei Navalny to Novichok nerve agent, claims activist just seeking publicity

Moscow urged by Western nations to 'transparently' reveal circumstances of opposition figure poisoning; Damascus faces calls for investigation over chemical weapon attacks

OPCW's conclusions match findings by labs in Germany, Sweden and France; Moscow has expressed doubts on the matter

Kremlin denies any part in incident, which German Chancellor Merkel called attempted murder

'My job now is to remain the guy who isn't scared,' says opposition leader, who is recovering in Berlin from nerve agent attack

But hospital notes it's too early to gauge the 'potential long-term effects' of Russian opposition leader's severe poisoning from nerve agent

Vil Mirzayanov, an ex-Soviet chemist who now lives in the US, was the first to reveal nerve agent in the early 1990s; he says he's sorry he 'took part in this criminal business'

Poisoned opposition leader and Putin critic jokes that breathing without a ventilator is 'strongly recommended'; doctors can't rule out long-term health issues