BANGKOK (AFP) - The United Nations said on Wednesday it was "seriously concerned" about growing restrictions on human rights activists in Thailand after a string of curbs on freedom of expression in the junta-ruled nation.

Since seizing power from the elected government in May, the Thai army has stifled dissent by hauling in anti-coup protesters, muzzling the media and threatening those found in breach of martial law with trial in a military court.

The UN Human Rights Office for South-east Asia (OHCHR) pointed to "a deteriorating environment for human rights defenders" a day after activists said they scrapped a debate about access to justice in post-coup Thailand due to pressure from the junta. "(The OHCHR) is seriously concerned about increasing restrictions on human rights defenders in exercising their rights to peaceful assembly and expression in Thailand," it said in a statement, calling on the country to honour its international human rights obligations.

On Tuesday, authors of a report on rights in Thailand since the coup said they received phone calls from military officers warning them against holding a discussion on the subject as it could be in violation of a ban on public gatherings.

The authorities also issued a letter in which they "asked for our co-operation" in cancelling the talk as "the situation is still abnormal", said Mr Pawinee Chumsri of The Thai Lawyers for Human Rights, who organised the event alongside Amnesty International Thailand and the Cross Cultural Foundation.

Anti-coup protesters have been detained, arrested and in some cases charged for opposing army rule.

Symbolic public readings of George Orwell's anti-authoritarian novel "1984" and the three-fingered salute from the "Hunger Games" movies were common in the weeks after the military takeover.

But public acts of resistance have since become more sporadic.

Junta chief Prayut Chan-o-cha, who was endorsed as Prime Minister two weeks ago, says he was forced to take power after months of protests against former premier Yingluck Shinawatra's government left 28 people dead and hundreds more wounded.

He has ruled out holding new elections before October 2015, despite international appeals for a return to democracy, vowing first to oversee reforms said to be aimed at cleaning up politics and society.