SEOUL, South Korea -- A fire spread flames and smoke through much of a South Korean hospital Friday morning, killing 37 people, mainly from suffocation, and injuring more than 100 others in one of the country's deadliest blazes in years, authorities said. The National Fire Agency had put the death toll at Sejong Hospital slightly higher, but said in a statement there had been some double counting.

The statement said the fire left 125 injured, 14 of them seriously.

The blaze started in Sejong Hospital's emergency room and had engulfed the first floor when firefighters arrived. They approached the second floor through windows to rescue trapped patients, said Choi Man-wu, a fire official in the southeastern city of Miryang, about 170 miles southeast of the capital, Seoul.

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He said smoke could have spread quickly through the building's staircase at the center, but the flames were extinguished before reaching the third floor. Videos from local TV networks showed black smoke billowing out of the building and engulfing its entire surface.

The cause of the fire wasn't immediately known. The hospital's operations were suspended after the fire.

Most of the dead had been hospitalized for respiratory diseases in an intensive-care unit on the second floor.

Hospital officials said an emergency room doctor, one nurse and one nurse assistant were among the dead.

The head of the foundation that operates the hospital also said in the televised briefing Friday afternoon that there were no sprinklers in the hospital, and they're not required by law.

Smoke rises from burning hospital in Miryang, South Korea, on January 26, 2018 Reuters

One-hundred-eleven patients and hospital staff members were transferred to six nearby hospitals, Yonhap said.

All 94 people being cared for in a nursing ward for the elderly were safely evacuated after the fire, some carried on the backs of firefighters, Choi said.

A patient is rescued from a burning hospital in Miryang, South Korea, on January 26, 2018 Kim Dong-min/Yonhap via REUTERS

Most of the deaths appeared to be due to suffocation, with only one suffering burns, said an official at the National Fire Agency who spoke on condition of anonymity because he wasn't authorized to speak to media. The identification of the dead was underway, he said.

President Moon Jae-in expressed regret over the blaze at an emergency meeting convened with his senior advisers. He ordered officials to provide necessary medical support to those rescued, find the exact cause of the fire and work out measures to prevent future fires, according to his spokesman Park Su-hyun.

South Korea is one of the fastest-aging countries in the world and has many nursing hospitals, which are preferred for elderly people who need long-term doctors' care.

Several recent fires in South Korea have been deadly.

In late December, 29 people were killed in a building fire in central Seoul, which was the country's deadliest blaze over the past decade before the hospital fire. Last weekend, a fire at a Seoul motel killed six people, and police arrested a man who allegedly set it ablaze in anger because he had been denied a room for being heavily drunk.

In 2014, a fire set by an 81-year-old dementia patient killed 21 at another hospital for the elderly.