The World Health Organization (WHO) said Tuesday that COVID-19 is on a slow but steady decline in most of western Europe but remains on the rise in Russia and parts of eastern Europe.

Speaking to reporters via video conference from WHO headquarters in Geneva, spokeswoman Dr. Margaret Harris said the number of new cases in the west remains "significant," and the decline is slow.

New cases in Russia, she said, along with other areas of eastern Europe, are still on the rise. She said the latest figures show Russia to have 414,878 cases of infection with 4,855 deaths.

Harris was asked about a recent study in the city of Wuhan, China - where the virus is believed to have emerged in December - in which Chinese authorities were said to have tested nearly 10 million people and found only 300, mainly asymptomatic cases of COVID-19.

She said much more research is needed globally to put this data into context. Harris said a study of that size "gives you a little piece of the puzzle, gives you a little bit of information. But it may be related to a setting, there's much more work that needs to be done around the world."

Harris said a WHO-led international mission to China and Wuhan earlier this year suggested that while asymptomatic transmission might play a part in spreading the disease, it did not appear to be the main "driver" of the outbreak.

As of Tuesday, the WHO reports more than 6 million confirmed cases of COVID-19 worldwide, including 373,548 deaths.

By region, the WHO says the Americas are the epicenter of the pandemic, with more than 2.8 million cases, followed by Europe (2.2 million), the Eastern Mediterranean (536,148), Southeast Asia (272,512), the Western Pacific (184,305) and Africa (108,121).