The death toll from the Halloween stampede in Seoul has increased to 154, including 26 foreigners, officials said on Monday, as the country mourned the deadliest tragedy in years.

The death toll could rise further, as 33 others remain in serious condition, according to officials at the Central Disaster and Safety Countermeasures Headquarters, adding that a total of 116 people sustained minor injuries, reports Yonhap News Agency.

All but one of the fatalities have been identified, officials said.

The 26 foreign victims include five from Iran, four each from China and Russia, two from the US, two from Japan, and one each from France, Australia, Norway, Austria, Vietnam, Thailand, Kazakhstan, Uzbekistan, and Sri Lanka, officials said.

Among those killed were one middle schooler and five high school students, all from Seoul, the Education Ministry said.

Five other middle or high school students have been injured, and two of them were being treated at hospitals, it said.

The tragedy took place on Saturday night when a massive crowd of Halloween partygoers packed a narrow 3.2-metre-wide alley in Seoul's entertainment district of Itaewon.

Some of them began to fall over, causing others to fall down like "dominoes" and pile up on one another.

It marked the deadliest stampede in South Korea's history and the worst disaster the country has seen since 2014, when the ferry Sewol sank in waters off the south coast and killed 304 people, mostly high school students.

The site of the accident is a downhill 40-metre back alley that links a busy restaurant district with a main street, where about six adults can barely pass at the same time.

Tens of thousands of people were visiting the area for Halloween festivities after the government lifted the outdoor mask-wearing mandate against the Covid-19 pandemic earlier this year.

A government probe in currently underway to determine the exact cause of the accident.

On Sunday, President Yoon Suk-yeol announced a week-long national mourning period starting Sunday and ordered the lowering of flags.

The government also set up a mourning altar in central Seoul to allow people to pay tribute to the victims.

On Monday, the President and the First Lady visited the altar where people have laid flowers and candies to mourn the victims.