BANGKOK - Thailand's King Bhumibol Adulyadej, the world's longest reigning monarch, is expected to leave hospital soon after nearly six weeks of medical treatment, his youngest daughter said Friday.

The health of the 81-year-old king is a sensitive topic in Thailand, where he is revered as a demi-god and has been stabilising force during his six politically turbulent decades on the throne.

"I wanted to tell everyone that the king's condition has significantly improved. I think soon he will regain his strength and return to the palace," Princess Chulabhorn told an audience in Bangkok.

"He has no fever and is continuing to get physical therapy. He can stand up and walk with a walker. He practises walking every day," added Chulabhorn, the youngest of the king's four children.

Rumours about the monarch's condition sent Thailand's markets stock plunging earlier this month and the government said on Thursday that four people had been arrested for spreading misinformation.

It is the third time that the princess has publicly commented on her father's health since the king was taken to Bangkok's Siriraj Hospital in mid-September with a lung infection and fever.

Last week he made his first television appearance since he was admitted to the clinic, being pushed in a wheelchair and carrying out religious ceremonies.

The king is widely viewed as playing a crucial role in maintaining stability in Thailand, especially amid the turmoil that has rocked the country since former prime minister Thaksin Shinawatra was toppled in a 2006 coup.