The head of the World Health Organization gave a boost Thursday to beleaguered South Korean officials and exhausted health workers, saying their efforts to contain a deadly MERS virus outbreak are reassuring and the public risk is low.

Margaret Chan told reporters in Seoul that it's a good sign the disease has so far been confined to a handful of hospitals and isn't spreading in the wider community. But she cautioned that much about MERS is unknown and there's still a need to strengthen monitoring and quarantine efforts.

Despite the optimism, there is still widespread worry here, and the discovery of new cases among people who managed to slip through quarantine measures has raised questions about officials' ability to control the outbreak.

Middle East respiratory syndrome has killed 23 people in South Korea, the biggest outbreak outside the region where it was first seen in 2012. More than 6,700 people are isolated at home and in medical facilities, and more than 160 have been diagnosed with MERS nearly a month after the outbreak started with a 68-year-old man who had traveled to the Middle East, according to Seoul's Health Ministry.

Most of the fatalities have been people with existing medical conditions, such as respiratory problems or cancer.

Officials say the outbreak has already peaked and could be defused by the end of the month. Experts say the coming week could test the government as the number of people isolated at home and in medical facilities approaches 7,000.

Critics, including Seoul Mayor Park Won-soon, say government officials accelerated the spread of MERS by failing to enforce tight control measures at Seoul's Samsung Medical Center, which was belatedly shut down over the weekend because it continued to be the main source of infections.

Dozens of patients, medical staff and visitors have been infected with MERS at the hospital, one of the country's biggest, and are believed to have contacted hundreds of other people before their conditions were confirmed.

The sheer size of the exposure at the hospital suggests the country could see another large wave of infections, according to Jacob Lee from the infectious disease department at Seoul's Kangnam Sacred Heart Hospital.