SHANGHAI, China - Shanghai partially restarted public transport Sunday and set out new classifications for Covid-19 risk areas, signaling a gradual reopening after nearly two months sealed off from the outside world.

China's largest city has been almost entirely locked down since April, when it became the epicentre of the country's worst coronavirus outbreak since the early days of the pandemic.

Unlike other major economies, Beijing has dug in its heels on a strict zero-Covid approach that relies on stamping out clusters as they emerge, though this has become increasingly difficult with the infectious Omicron variant.

But as new infections have slowed, Shanghai has cautiously eased restrictions, with some factories resuming operations and residents in lower-risk areas allowed to venture outdoors.

Four of the city's 20 subway lines restarted Sunday along with some road transport, with officials announcing last week that it would provide a "basic network covering all central urban areas."

Those who take public transport will have to show a negative Covid test within 48 hours of their journey and have a "normal temperature", they added Saturday.

Shanghai will also classify areas as high, medium or low-risk after May 31, city health official Zhao Dandan told a press briefing on Sunday.

Districts with 10 or more reported Covid cases -- or at least two community infections -- will be considered "high-risk" while areas with no positive cases for 14 days will be deemed "low-risk", Zhao said.

Medium or high-risk areas face lockdowns of two weeks.

The new system appears to set the stage for a degree of movement comparable to other cities, a shift from tough current measures in which even residents of lower risk areas have faced tight restrictions.

-Back under lockdown-
But despite broader attempts to ease those restrictions, the city's central Jing'an district was back under lockdown on Sunday, according to an official notice.

Jing'an will undergo three consecutive rounds of mass Covid testing from Sunday and residents are not to leave their homes during this period, a WeChat notice said.

"Exit permits that have been issued will be suspended," the notice added Saturday, while assuring residents that "victory is not far away".

The city of 25 million residents reported more than 600 Covid cases on Sunday, 570 of them asymptomatic, according to National Health Commission data.

But restrictions continued in other Chinese cities with Covid cases, including the capital Beijing, which has already banned dining out and forced millions to work from home.

As of Saturday, nearly 5,000 people in Beijing's Nanxinyuan residential compound had been relocated to quarantine hotels after 26 new infections were discovered in recent days, state media reported.

Fears have run high that the city may take a similar approach to Shanghai, where the lockdown has denied many adequate access to food and medical care.