One more fatality was confirmed from the Halloween stampede in Seoul's Itaewon district, raising the death toll from the tragedy to 156, officials said on Tuesday.

A 20-year-old Korean woman was pronounced dead on Tuesday morning while under treatment in serious condition, according to the Central Disaster and Safety Countermeasures Headquarters.

A total of 29 people are still in serious condition, while 122 others had sustained minor injuries, Yonhap News Agency quoted the officials as saying.

Of those killed, 101 were female, and victims in their 20s numbered 104, followed by 31 victims in their 30s.

There were also 26 foreign victims including 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.

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.

Meanwhile, the police have also intensified its investigation into the disaster and were looking into testimonies from dozens of eyewitnesses and surveillance camera footage to reconstruct the circumstances of the deadly incident.

Nam Gu-jun, chief of the National Office of Investigation, said a 475-member special investigative team had interviewed 44 witnesses as of Monday and secured 52 pieces of surveillance camera footage in 42 places around the scene to determine the exact cause of the accident.

Videos posted on social media were also being scrutinized, he said.

President Yoon Suk-yeol declared a week-long mourning period till Saturday, and mourning altars have been set up nationwide to allow people to burn incense and pay their respects to those killed.