Russia has temporarily stopped vaccinating new volunteers in its Covid-19 vaccine trial due to high demand and a shortage of doses, a representative at the firm running the study said on Thursday, in a setback for Moscow's ambitious plan to roll out the shot.

"It's related to the fact that there's colossal demand for the vaccine and they are not producing enough to keep up," said the representative of Crocus Medical, the contract research organisation that is helping run the trial in Moscow together with Russia's health ministry.

According to provisional information, vaccinations will restart by around 10 Nov., he said.

Staff in eight of the 25 Moscow clinics hosting the trial told Reuters the vaccination of new participants in the Phase III study had been temporarily paused, with several citing strong demand and saying they had run out of doses.

The Moscow City Health Department, which oversees the 25 Moscow clinics where the trial is taking place, did not respond to a request for comment.

In response to questions about the temporary halt of the trial and low supply of doses, Alexei Kuznetsov, aide to Russia's health minister, said the human trial of the vaccine continued.

"The target of 40,000 vaccinated volunteers will be met," he said.

Moscow's Gamaleya Institute, which developed the vaccine and is also manufacturing it, directed questions to the health ministry.

Earlier on Thursday, President Vladimir Putin said Russia was facing challenges scaling up production of its main Covid-19 vaccine due to problems with equipment availability, but hoped to start mass vaccinations by the end of the year.