Shanghai authorities have said tough restrictions would remain in place for now even in districts which managed to cut Covid-19 transmission to zero, prolonging the agony for many residents who have been stuck at home for most of this month.

That sober assessment, prompted by an unexpected rise in the number of cases outside quarantined areas, came after health officials earlier in the week had fuelled hopes of some return to normal by saying that trends in recent days showed Shanghai had “effectively curbed transmissions”.

An official from the Chongming district, an outlying island area, said most curbs would be kept in place, although it has reported zero cases outside quarantined areas and 90 per cent of its 640,000-or-so residents were now in theory allowed to leave their homes.

“For those in prevention areas, we have to continue to ensure that they don't become 'free to fly' areas,” deputy governor Zhang Zhitong said, referring to neighbourhoods which meet the criteria for residents to be allowed to go outside.