MOSCOW/LONDON, Oct 7 - Russia's health regulator said on Wednesday there were no risks of an epidemic breaking out in Siberia after a plague institute worker there died, as the World Health Organization pressed Moscow for more information about her cause of death.

Russian authorities say the 28-year-old laboratory worker died last Friday from pneumonia of an as-yet unestablished origin, in a case that has prompted international concern including from US President Donald Trump. Around 200 people were initially placed in quarantine.

Asked on Wednesday whether the death was caused by a bioweapon, Trump told reporters: "Well, we don't think so. We're going to find out soon enough, but we don't think so." He said Russia was not saying much about the situation but "they do say they have it very much under control," adding he had set up a call with Russian President Vladimir Putin.

"They're sharing (information) a little bit with our people. We expect them to share it a lot with our people," said Trump, speaking in the Oval Office.

Global health officials - as well as the world's media - have been particularly alert to pneumonia cases of undetermined cause since the COVID-19 pandemic, which began with a cluster of fast-spreading fatal respiratory illness in China. However, the cause of individual pneumonia cases can often not be determined.

US Centers for Disease Control and Prevention Director Erica Schwartz said at an event the agency was prepared to monitor and screen travelers although the risk to the US was extremely low The WHO initially said it estimated the risk of the Russia case to the public as low, based on what had been made known, but told a press conference on Wednesday it could not make a fuller risk assessment due to insufficient information from the Russian authorities.

Rospotrebnadzor, Russia's health watchdog, said on Wednesday it was winding down precautionary measures it had put in place after concluding there was no risk of an epidemic and that it had shared its latest information with the WHO.

No cases of plague had been detected among anyone who had come into contact with the deceased worker, Darya Shipilova, it said, and 90% of those placed in quarantine had been released after nearly 5,000 lab tests had failed to detect any dangerous pathogens.

It said Shipilova had been vaccinated and had therefore been "immune to the dangerous infections with which she worked". It did not say, however, what had caused the pneumonia which killed her or how she fell ill.

Tedros Adhanom Ghebreyesus, the director-general of the WHO in Geneva, said on Wednesday his organisation needed more information from Russia to understand what had happened.

"We would like more information to understand which laboratory tests were conducted, which pathogens were evaluated, and if this individual didn't die from plague, then how did she die?" Maria Van Kerkhove, director of the WHO's department of epidemic and pandemic management, told a virtual press conference.

Tedros said the WHO had offered assistance to Russia and also repeated a request for Russia to verify "media reports of a second employee with pneumonia of undetermined cause".

The Kremlin said Russia had properly informed the WHO about the incident in line with established procedures.

When asked about the WHO's request to respond to reports about a second person in Siberia allegedly falling ill, Kremlin spokesman Dmitry Peskov urged caution when it came to using media reports as a reference for what was happening.

"There is a great deal of media coverage, and the vast majority of what the media are reporting about this situation is false information," said Peskov.

He advised the media and others to rely on information provided by the health regulator. It did not immediately respond to a Reuters request for comment on Wednesday about the WHO's questions.

Some Western media outlets have reported — citing unnamed "Russian media" — that a second person in Siberia had died, saying they had died of plague.

Reuters was unable to find any such stories in Russian media or any evidence that a second person in Siberia had died although some bloggers on social media were making — without evidence — such assertions.

After the press conference, the WHO sent Reuters a link to a story about a potential second case by the Russian media organization starhit.ru, a celebrity gossip outlet, and a follow-up from British newspaper the Daily Mail.

Tedros said it was important to be transparent based on the lessons from COVID, adding, "We don't have any mandate actually to see whether this media is credible or not." Russia has not formally responded to the reports about a second person dying, Tedros said, adding that he had heard only informally that Russia had denied those reports.

Tedros' call for more information echoed that of US Secretary of State Marco Rubio, who has also said Moscow needs to share more information about the incident.

Pneumonia, a lung infection, can have a variety of causes including plague as well as other bacterial infections or viral respiratory diseases, such as flu or COVID-19.