A chain of coronavirus infections traced to Seoul's clubbing district Itaewon has grown to seven levels, causing uneasiness among parents as over two million students go back to the classroom today in the second phase of school reopening.

The number of cases in the cluster has risen to 255 as of noon yesterday, including 127 in Seoul and 45 in Incheon.

The total tally stands at 11,225, with 269 deaths.

Even with mass testing and aggressive contact tracing, the health authorities have not been able to stop the spread of the virus that keeps claiming more victims, including 19 reported yesterday.

E-commerce giant Coupang - the Amazon of South Korea - closed its logistics centre in Bucheon, west of Seoul, after 11 cases were found to be linked to the facility yesterday, including nine employees.

Reports said the infections have been traced back to a 25-year-old male teacher from Incheon city who visited a club in Itaewon over a long weekend earlier this month.

As of yesterday, the infection chain related to him has grown to seven levels, involving 90 cases.

He first spread the virus to his students in Incheon, who visited a coin karaoke room, infecting it. A cab driver who went singing there transmitted the virus to a birthday party at a buffet restaurant in Bucheon, where it spread among attendees.

The virus was then carried to two other restaurants by diners, one of whom further infected a family member - the seventh in the chain.

Mr Kwon Joon-wook, deputy director of the Korea Centres for Disease Control and Prevention, voiced concern yesterday over the growth in infection figures, noting that "one person can trigger many transmissions".

He also stressed the importance of cooperation with epidemiological investigations, referring to how the Incheon teacher had hindered contact tracing efforts by initially lying that he was unemployed.

Over 83,000 people have been tested in relation to the Itaewon cluster, which emerged on May 6.

The government is pushing for a return to normal life with "everyday life quarantine" that includes rules such as wearing masks and frequent hand washing.

Some 450,000 high school seniors returned to the classroom last week in the first phase of school reopening. The second phase begins today, with 2.37 million students, including kindergarteners and elementary school pupils, set to return.

Many parents have voiced concern over sending young children back to school, questioning if schools are prepared to prevent a possible outbreak.

An elementary school in Seoul delayed its reopening after a student tested positive for the virus yesterday. Another elementary school, in Bucheon, also postponed its reopening after a teacher tested positive.

To ease parents' concerns, President Moon Jae-in said the government will "make every effort" to ensure safety in schools.

"The government is fully aware of the worries of parents and we're all on the same page when it comes to protecting our children," he said.