The Chinese financial hub of Shanghai said three people infected with Covid-19 died on Sunday, the first time during the current outbreak that it reported deaths among coronavirus patients.

The people who died included two women aged 89 and 91, and a 91-year-old man, the city said, adding that they all had a variety of underlying health complaints such as coronary heart disease, diabetes and high blood pressure.

City officials said only 38% of over-60s in Shanghai are fully vaccinated.

The city is now due to enter another round of mass testing, which means a strict lockdown will continue into a fourth week for most residents.

The city reported 19,831 new daily asymptomatic Covid-19 cases on April 17, down from 21,582 on the previous day. New symptomatic cases stood at 2,417, down from 3,238.

The city has conducted more than 200 million nucleic acid tests since March 10 in a bid to curb China's biggest COVID-19 outbreak since the coronavirus was first discovered in Wuhan in late 2019.

Since the discovery of an Omicron-led outbreak three weeks ago, the city has been under strict lockdown, which has angered residents.

Millions have been confined to their homes, with anyone testing positive being sent to quarantine centres.

In recent weeks many have taken to social media to complain about the restrictions and the lack of food supplies.

People have had to order in food and water and wait for government drop-offs of vegetables, meat and eggs, and analysts say many are running low on supplies.

The lockdown extension has overwhelmed delivery services, grocery shop websites and even the distribution of government supplies.

But with more than 20,000 new cases a day, authorities are struggling. The city in recent weeks has converted exhibition halls and schools into quarantine centres, and set up makeshift hospitals.

The recent surge in cases in China, although small compared to some countries, is a significant challenge to China's "zero-Covid" strategy, which uses swift lockdowns and aggressive restrictions to contain any outbreak.

The policy sets China apart from most other countries which are trying to live with the virus.

But the increased transmissibility and milder nature of the Omicron variant has led to questions over whether the current strategy is sustainable in the long run.