Among the 23 new cases, 17 were infected at the Samsung Medical Centre in southern Seoul, where the country's first patient was diagnosed,the Health Ministry said. All the infections so far have been restricted to hospitals, with transmissions between patients, staff and their families.

There were no additional cases from another hospital that produced the first wave of infections with 37 patients.

One of the new cases was a 16-year-old student hospitalised on May 27 for another disease, the Education Ministry said, in the first case involving a teenager.

Given the period of time he had been in hospital, the ministry stressed it was "not possible" that he had infected any classmates at school.

A team of experts from the World Health Organisation (WHO) was due to begin work on Tuesday to evaluate the response to the outbreak, including why it had spread so fast and advise on further measures.

The South Korean culture of families looking after their loved ones at hospitals may have been part of the reason for it to spread within health-care facilities, WHO Director-General Margaret Chan told Yonhap news agency.

All known South Korean infections have taken place within healthcare facilities, where it is common for family and friends to make lengthy visits, sometimes around the clock.

The WHO has not advised any travel restrictions.

Criticised for its initial response to the outbreak, the government on Sunday vowed "all-out" efforts to curb the further spread of the virus, including tracking the mobile phones of those under house quarantine to ensure they stay home.

Hundreds of public events, school trips and sporting events have been cancelled.

Health authorities said they were expecting to see more new cases of those who had been infected from the Samsung hospital in recent weeks.

The outbreak has triggered widespread public concern in South Korea, with 2,300 people placed under quarantine orders and nearly 1,900 schools - mostly in Seoul and surrounding Gyeonggi province - closed down.

The Samsung Medical Centre - one of the South's largest hospitals - has placed nearly 900 patients and medical staff under observation.

More than 20 countries have been affected by Mers, with most cases in Saudi Arabia.

The virus is considered a deadlier but less infectious cousin of severe acute respiratory syndrome, which killed hundreds of people when it appeared in Asia in 2003.

Reflecting public concern among parents, 1,869 schools across the country were due to be closed on Monday, the Education Ministry said.

All school trips from Singapore to South Korea had been postponed or cancelled, Singaporean media reported over the weekend, citing the Ministry of Education.

The quarantine office at Japan's Narita Airport, which serves Tokyo, said announcements were being made on planes from South Korea that anybody who might have been in contact with a Mers patient or been in a hospital with Mers patients needed to report to quarantine officials.

The country's first patient returned from Saudi Arabia in early May, officials have said.

First identified in humans in 2012, Mers is caused by a coronavirus from the same family as the one that triggered severe acute respiratory syndrome.

South Korea's new cases bring the total globally to 1,236, based on WHO data, with at least 445 related deaths.

With the economy already flagging, the Mers outbreak is adding pressure for another interest rate cut in South Korea, possibly as soon as the central bank's next policy meeting this week. The finance minister has said there is no need for a supplementary budget.