Muscovites are being urged to take part in a new testing phase of Russia's coronavirus vaccine.
The "post-registration clinical" trial should last six months and involve around 40,000 people, Moscow's mayor said in a statement.
The Russian Direct Investment Fund, which bankrolled the "Sputnik V" vaccine, said it would facilitate similar trials in five other countries, but didn't name them.
But international medical experts expressed unease, calling Russia's fast-tracked approval and failure to share any data supporting claims of the vaccine's efficacy a major breach of scientific protocol.
At the time, Russian officials said mass vaccination could begin as early as October. But Moscow's call for volunteers to take part in trials would appear to cast doubt over this timetable.
Scientists at the World Health Organization said last week that although they had begun discussions with Russia about its vaccine, they had not yet received any detailed data about it.
Speaking to Euronews, the former associate commissioner of the US Food and Drug Administration (FDA), Peter Pitts, raised concerns about Russia's vaccine announcement.
"There's no data, there's no transparency, there's no FDA [Food and Drug Administration] in Russia [and] they've got a history of approving drugs and vaccines with little or no testing," he said.
Experts warned that using an untested vaccine that has not yet proven to be safe or effective could ultimately undermine the response to the pandemic and cause more distrust among people about whether or not to be vaccinated.
There is no certainty as to when an effective vaccine against coronavirus will be approved.
Currently, there are 167 candidate vaccines being studied, 32 of which undergoing human trials, including eight which have moved to phase three, with large-scale testing.