Hong Kong (CNN)-- A man who completed 21 days of mandatory quarantine upon returning to China from overseas has been identified as the likely source of a new outbreak, raising questions over the sustainability of the country's stringent zero-Covid strategy.

The fresh outbreak in Fujian province on China's southeastern coast has infected more than 60 people, including 15 elementary school pupils. It emerged just two weeks after China contained its worst coronavirus flareup in more than a year, highlighting the increasing challenge posed by the highly contagious Delta variant -- even to a country with some of the world's strictest, most far-reaching containment measures.

The latest infections were first detected in two young brothers during a routine Covid test at a elementary school in Xianyou county, Putian city, on Thursday. Another student and three parents tested positive the next day, the Putian municipal government said at a press conference Friday.

Experts advising the government have pointed to one of the parents -- a father who recently returned from Singapore -- as the likely source of the outbreak, despite the man having completed a lengthy quarantine on arrival in China.