SEOUL (AFP) - South Korea on Wednesday announced its 20th death from the Middle East Respiratory Syndrome (Mers) virus as criticism grew of efforts to contain the outbreak, with alarming reports of new cases slipping through a quarantine that already affects thousands.

A 54-year-old woman died of Wednesday morning after being diagnosed on June 5, the health ministry said.

It also reported eight new patients including four who had been infected at Samsung Medical Centre in Seoul, considered the epicentre of the outbreak.

This took the total number of infections including those who have died to 162, the largest outbreak outside Saudi Arabia.

Seventeen patients are in critical condition, the ministry said, adding 19 had recovered and been released from hospital.

The number of new infections had fallen for three days in a row from 12 on Friday to four on Tuesday, when the ministry said it was cautiously optimistic the worst was over.

But the latest number dashed such hopes, as a health official raised concerns over more new infections beyond the Samsung hospital.

"We think that more new cases can sporadically occur en masse in hospitals other than Samsung," senior health ministry official Kwon Joon-Wook told reporters.

The government aims to curb outbreaks in 11 hospitals considered major epicentres - including Samsung - by the end of this month, he added.

Critics say the government's handling of the crisis shows it has done little to improve lax overall public safety standards since the Sewol ferry disaster in April 2014.

Almost all infections so far have been restricted within hospitals.

But several patients diagnosed in recent days were not among those put in quarantine.

One patient reported Tuesday in the southeastern city of Daegu developed symptoms on Saturday but continued his normal activities, including going to work and taking a trip to a public bathhouse.

The 52-year-old public servant, currently in isolation, visited his mother in late May while she was a patient in the Samsung hospital, and his sister was diagnosed with MERS on June 10, the Daegu city council said Wednesday.

Another patient - a 55-year-old ambulance driver at the Samsung hospital - continued to go to work via subway for days after developing symptoms including fever on June 2. He was officially diagnosed last Friday, the health ministry said.

He came into contact with nearly 500 people while developing symptoms including 160 patients at the Samsung hospital, it said, adding most had been placed under observation.

Dong-A Ilbo newspaper slammed the government for "unrealistically" playing down the magnitude of the outbreak.

"All the optimistic words and predictions from health authorities have turned out to be wrong so far," it said in an editorial.

"We are concerned whether (health minister) Moon Hyung-Pyo ... will ever be able to get the situation under control."

Hospital at epicentre

A staff member at Samsung hospital is also among the eight new cases announced Wednesday, though the health ministry said it was unclear where the 33-year-old had been infected.

About 80 patients, visitors and workers have contracted the virus at the hospital - one of the country's largest, which suspended most operations on Sunday to stem the further spread of the disease.

Staff members at two other hospitals were also diagnosed, as well as a patient in the city of Hwaseong, 43 km south of Seoul.

The number of people who were exposed to patients and put under quarantine - either at state facilities or at home - has jumped by 922 to more than 6,500.

As public alarm grew, the number of subway and bus passengers plunged by more than 20 per cent in the capital Seoul, the city council said Wednesday.

The virus appeared in South Korea on May 20 when a 68-year-old man was diagnosed after returning from a trip to Saudi Arabia.

Since then it has spread at an unusually rapid pace, sparking widespread alarm.

There is no vaccine for Mers which has a mortality rate of 35 per cent, according to the World Health Organization.