The health of Thailand's 88-year-old King Bhumibol Adulyadej, the world's longest reigning monarch, has "overall not yet stabilised", the palace said in a statement today.

Earlier, the government, in a clear reference to the king's health, urged people not to listen to rumours on social media about "situations", saying they should await official announcements.

The statement from the palace follows one on Sunday in which it said the king was in an unstable condition after receiving haemodialysis treatment, which is used to cleanse the blood of toxins, extra salts and fluids.

"On Oct 11 his blood pressure lowered and breath quickened," the palace said, adding that the king's health has "overall not yet stabilised".

Several hundred people gathered today at Bangkok's Siriraj hospital, where the king has been for much of the past year, to pray for the widely revered monarch.

During his seven decades on the throne the king, who is seen as a unifying force, has intervened when events threatened to plunge Thailand into crisis.

He has been treated for various ailments over the past year and was last seen in public on Jan 11, when he spent several hours visiting his palace in the capital, Bangkok.

A Reuters reporter at the hospital said all four of the king's children, including Crown Prince Maha Vajiralongkorn, visited him.

Official engagement cancelled

Prime Minister Prayuth Chan-ocha, who is also head of the ruling junta, cancelled an official engagement in the eastern province of Chonburi this morning.

"The prime minister did not undertake his official duties in Chonburi province today in order to prepare for an audience with His Royal Highness the Crown Prince for a routine presentation on the government's work in progress," the Foreign Ministry said in a statement.

"The government spokesperson has urged for the Thai public to rely on official announcements for an update on situations, rather than uncorroborated information in social media circles," the ministry said, without referring to the king's health.

US Assistant Secretary of State Daniel Russel said the US-born king has been a "true friend of the United States".

"He's a revered figure in Thailand and he's an admired figure in the United States and we hope his situation will stabilise," Russel told reporters in Washington.

Thailand has long been an ally of the US, for decades united in opposition to the spread of communism in South-East Asia.

The Stock Exchange of Thailand closed down 2.5 percent, its lowest since the end of May, after falling as much as 6.9 percent at one point to its lowest since March 1, led by shares in the aviation and tourism industries.

Today's palace statement was the third health bulletin this month after the palace said on Oct 1 the king was recovering after a respiratory infection.

Laws protecting the royals from insult make it a crime to defame, insult or threaten the king, queen, heir to the throne or regent.

- Reuters