The United States has not lost Thailand to China, Washington's envoy to Bangkok said Monday, despite acrimony between the two allies and a palpable shift by the kingdom's junta towards its giant northern neighbour.

"I don't spend a lot of time, I don't spend any time, saying to Washington here's how we get Thailand back. We haven't lost Thailand," ambassador Glyn T. Davies told reporters in Bangkok.

"I think it's a good thing for Thailand to have a good relationship with China," he added.

Thailand has been one of Washington's staunchest military allies in Southeast Asia and could have expected to see that relationship blossom under US President Barack Obama's "pivot" to Asia.

But the May 2014 coup, the second in the last decade, and the junta's subsequent rights crackdown has strained those ties.

Thailand's latest crop of military leaders have been embraced by Beijing, a warmth that has been reciprocated.

The junta has also begun forcibly deporting critics of Beijing back to China, some of whom had recognised refugee status, sparking condemnation from the United Nations and other western allies of the kingdom.

Davies has only been in the job nine weeks but has already ruffled feathers in a nation where ultra-royalist generals currently hold political sway.

On Friday an arch-royalist Thai monk led a protest outside the US embassy -- despite a current junta ban on political protests -- sparked by recent criticism Davies had made of the kingdom's controversial and draconian lese majeste law.

Thailand's revered but ailing King Bhumibol Adulyadej, 87, is protected by one of the world's strictest royal defamation laws, prosecutions under which have surged since the military seized power.

On Wednesday Davies hit out at "the lengthy and unprecedented prison sentences handed down by Thai military courts against civilians" for breaching lese majeste.

The comments were no major policy shift from Washington, but public criticism of the law is rare.

On Friday junta chief Prayut Chan-O-Cha hinted that trade ties between the two nations could be affected if Davies repeated such remarks.

Asked for a response to Prayut's threat, Davies on Monday replied: "We hope the trade issue is looked at on its merits exclusive of politics and geopolitics."

"If political leaders want to equate the two that's up to them, up to him," he added.

Davies said he also hoped to take part in "Bike for Dad", an mass cycle ride next month organised by the Crown Prince Maha Vajiralongkorn to mark his father's 88th birthday.

King Bhumibol has been largely confined to hospital in recent years.

Davies insisted the United States has a "special attachment" to Thailand's king.

"We love King Rama 9," he said, using his official title.

"So out of respect for the king, since we're all pulling for him, I think I will ride in Bike for Dad," he said.