SEOUL, South Korea — The number of South Koreans infected with Middle East respiratory syndrome increased to 87 on Monday after the government identified additional cases. It also said that an 80-year-old man had died, bringing the number of deaths linked to the virus in South Korea to six.

The spread of the virus in South Korea, which in a matter of days has become the worst-afflicted country besides Saudi Arabia, has caused concern throughout Asia. The health authorities in Hong Kong raised their three-stage response level on Monday from “alert” to “serious,” which means ports of entry will exert tighter arrival controls. The Hong Kong Center for Health Protection posted an advisory about the raised response on its website, urging people to “avoid unnecessary travel” to South Korea.

The World Health Organization has not issued any special warnings for South Korea and has reaffirmed its confidence that the South Korean health system can overcome the breakout of the disease, known as MERS.

Nonetheless the organization is closely monitoring the progression of the disease, which causes high fever and pneumonia-like symptoms, and is spread through close contact among humans. It is believed to have first been spread to humans from camels in Saudi Arabia three years ago and has now been seen in at least 25 countries.