BEIJING, China -- China reported 3,393 new Covid-19 cases Sunday, the National Health Commission said, more than double the previous day, as the country faces its gravest virus outbreak in two years.

A nationwide surge in cases has seen authorities close schools in Shanghai and lock down several northeastern cities, as almost 19 provinces battle local outbreaks of the Omicron and Delta variants.

The city of Jilin has been partially locked down, with hundreds of neighbourhoods sealed up, an official announced Sunday.

China, where the virus was first detected, has maintained a strict 'zero-Covid' policy enforced by swift lockdowns, travel restrictions and mass testing when clusters have emerged.

But the latest flare-up, driven by the highly transmissible Omicron variant and a spike in asymptomatic cases, is challenging that approach.

Residents of Jilin have completed six rounds of mass testing, local officials said. On Sunday the city reported over 500 cases of the Omicron variant.

The neighbouring city of Changchun -- an industrial base of nine million people -- was locked down Friday.

" reflects that the spread of the Omicron variant is hidden, highly contagious, rapid and difficult to detect in the early stages," said Jilin provincial health official Zhang Yan at a Sunday press briefing.

"It also reflects the rapid rise of the virus situation in individual regions and the lack of expansion capacity of medical resources, resulting in limited centralised admission and treatment in a short period of time." The mayor of Jilin and the head of the Changchun health commission were dismissed from their jobs Saturday, state media reported.

The smaller cities of Siping and Dunhua, both in Jilin province, were locked down Thursday and Friday, according to official announcements.

Hunchun city, bordering Russia and North Korea, was locked down March 1, local authorities said.

Three makeshift hospitals were built in the city to treat Covid-19 patients, Xinhua reported on Saturday.