South Korea's state heritage body apologised Thursday for apparent structural problems at a landmark monument that only recently re-opened after a five-year, multi-million dollar restoration.

Seoul's 600-year-old Namdaemun (South Gate), listed as "National Treasure Number One" and a source of immense cultural pride, was burned pretty much to the ground on February 10, 2008.

The largely wooden monument re-opened to great fanfare in May this year, after a $23 million project that involved scores of highly-skilled artisans using traditional methods to restore the gate to its original glory.

Raw materials for decorative paints had to be imported from Japan, since Korean specialists had lost the art of making them in the traditional fashion.

In recent weeks, however, media reports have highlighted cracks appearing in the gate's main pillars and paint peeling from the hand-coloured decorations.

"We sincerely apologise for causing trouble to the people ... and will do our best for the thorough and perfect preservation of the restored gate," the Cultural Heritage Administration said in a statement.

It promised an investigation to determine whether the problems were caused by shoddy workmanship.

The landmark was one of four gates built to protect the city when it was the capital of the Joseon Dynasty, which ruled the Korean peninsula from 1392 until the Japanese occupation in 1910.

The 2008 fire was started by a lone, disgruntled 69-year-old arsonist with some paint thinner and a cigarette lighter.

The gate's destruction sent shock waves through the country, with sorrowful Seoul residents swarming around the charred ruin, laying flowers and writing grieving messages.

The arsonist was eventually sentenced to 10 years in prison.