Thailand's junta chief said on Tuesday he would lift martial law but only after replacing it with a new order retaining sweeping powers for the military.

Critics said the move would "deepen dictatorship" in the kingdom.

Prime Minister Prayut Chan-O-Cha said he had asked the ailing 87-year-old King Bhumibol Adulyadej for permission to lift the controversial law, which would then be replaced with special security measures.

The former army chief imposed martial law and seized power last May following the ousting of Yingluck Shinawatra's democratically elected government after months of often violent street protests.

It was the latest twist in a decade of political conflict broadly pitting a Bangkok-based middle class and royalist elite -- backed by parts of the military and judiciary -- against pro-Shinawatra urban working-class voters and farmers from the country's north.

Speaking to reporters Tuesday, Prayut said a new order to replace martial law would be "issued very soon".

Junta officials said the measures, which have yet to be fully defined, would create a "better atmosphere" in the kingdom, where dissent has been strongly suppressed since the military takeover.

But human rights groups expressed alarm that an executive order could allow Prayut to wield even greater powers.

Major General Sunsern Kaewkumnerd, a junta spokesman, told reporters Prayut felt the decision was necessary because "foreign countries were concerned over our use of martial law".

Some businesses and tour operators have also called for the controversial law to be lifted.

A spokeswoman for the US embassy in Bangkok said they would welcome the lifting of martial law if it led "to the full restoration of civil liberties".

Under the law the army has been able to prosecute those accused of national security and royal defamation offences in military courts with no right of appeal.

The media, meanwhile, has been muzzled while political gatherings of more than five people are banned.

Military to retain key powers

In his first public comments on what might replace martial law, Prayut clearly indicated that the military would retain significant powers.

The former army chief said he would use Article 44 of the junta's interim constitution to issue a new order protecting Thailand's security.

The article grants Prayut power to make executive orders on national security issues without having to go through the military-stacked parliament.

Prayut said military courts would still be used for security offences but convictions could now be appealed to higher tribunals.

Security forces would continue to be able to make arrests without a court warrant, he added.

Prayut did not say, however, whether cases under Thailand's royal defamation law -- one of the world's strictest -- would continue to be prosecuted through military courts, or whether the current ban on political gatherings would be lifted.

A joint statement signed by Thai Lawyers for Human Rights and seven other rights groups warned that using Article 44 would grant Prayut "absolute powers... over the legislative, the administrative and the judiciary".

"The world won't be fooled. This is a deepening of dictatorship," added Sunai Phasuk of Human Rights Watch.

But Puangthong Pawakapan, a Thai politics expert at Chulalongkorn University, said lifting martial law might alleviate military excesses in the rural north where support for the Shinawatras is strongest.

"These abuses in the provinces, hopefully, should be lessened once the law is lifted," she told AFP.

Prayut has vowed to return power to an elected civilian government, but only once reforms to tackle corruption and curb the power of political parties are codified in a new constitution.

Critics say those reforms are aimed at neutering the power of Yingluck and her brother, ousted prime minister Thaksin Shinawatra, ensuring that they and parties linked to them can never take office again.

Rights groups say basic freedoms have been severely eroded since the military took over and lese majeste legislation has been increasingly used to stifle political opposition.