Authorities in Shanghai have again tightened anti-virus restrictions, just as the city was emerging from a month of strict lockdown due to a COVID-19 outbreak.

Notices issued in several districts said residents were ordered to stay home and are barred from receiving nonessential deliveries as part of a 'quiet period' lasting at least until Wednesday. The tightened measures could be extended depending on the results of mass testing, the notices said.

'Thank you for your understanding and cooperation. Together we can lift the lockdown at an early date,' said one notice issued in the city's Huangpu district and posted online.

It wasn't clear what prompted the renewed tightening, with numbers of new COVID-19 cases in the city continuing to fall.

Shanghai on Monday reported 3,947 cases over the previous 24 hours, almost all of them asymptomatic, along with 11 deaths. Authorities have been gradually lifting isolation rules on the city's 25 million residents, but the new orders appear to be returning to conditions at the early stage of the outbreak.

Shanghai originally ordered mass testing along with a limited lockdown, but extended that as case numbers rose. Thousands of residents have been forced into centralized quarantine centers for showing a positive test result or merely having been in contact with an infected person.

Two Shanghai residents reached through social media said they'd had no prior notice of the new restrictions, which they were told could last for up to a week.

'We're unprepared,' said Zhang Chen, a researcher with a technology company. 'I packed my luggage thinking it would be my turn next' to be taken to a quarantine facility.

'I don't know what will happen in May, but after the lockdown, I think I'll need psychological help,' Zhang said.

A marketing professional in the western Pudong district said quality of life has been declining even as living expenses continue to rise under lockdown.

'Every time, they say lockdown will be eased after a few days, but there seems to be no end,' said the woman, who asked that she be identified only by her surname, Lu, to avoid repercussions from authorities who have cracked down heavily on dissent.

'All aspects of work are affected. I don't know when it will be time for the lockdown to come to an end,' Lu said.

In Beijing, authorities closed down the largest city district, with residents told to stay home and stores closed. Beijing has ordered daily testing of all residents, closed parks and other leisure venues and limited restaurants to takeout business only.

The usually bustling Sanlitun area crammed with restaurants, boutiques and an Apple store was all but deserted. Despite that, retiree Yang Xiaochang said Beijing appeared to be far better prepared to weather the surge than its southern cousin.

'Even though at the beginning there were some panic buying ... Beijing will not be like that,' Yang said, referring to Shanghai.

Still, companies and investors worry the ruling Communist Party's 'zero-COVID' strategy that closed most businesses in Shanghai and other industrial centers is disrupting global trade and activity in autos, electronics and other industries.

China's export growth tumbled in April as global demand weakened, adding to pressure on the world's second-largest economy.

Exports rose 3.7% over a year earlier to $273.6 billion, down sharply from March's 15.7% growth, customs data showed Monday. Reflecting weak Chinese demand, imports crept up 0.7% to $222.5 billion, in line with the previous month's growth below 1%.