Human rights groups describe the policy announced by Thailand's Internal Security Operation Command as "inhumane".

"Rather than sympathy and support for those at risk, Thailand is preparing with the back of its hand," said Sunai Phasuk, a senior researcher at Human Rights Watch.

Analysts expect many of 436,000 Rohingya who have fled Myanmar's northern Rakhine state since August to be recruited by people-smugglers and take to the seas in unsafe boats to try to reach Thailand, Malaysia, Indonesia and eventually Australia.

The UN has described attacks on Rohingya in Rakhine as "textbook ethnic cleansing".

Humanitarian aid has also been severely restricted to 140,000 other Rohingya displaced by previous religious unrest and confined to squalid camps near the state capital Sittwe.

The UN children's agency, UNICEF warned on Tuesday the situation, in what has grown into one of the world's biggest refugee camps in Bangladesh, could become a catastrophe with 14,420 Rohingya children there already classified as severely or acutely malnourished.

The boat people exodus is expected on the Andaman Sea at the end of the monsoon season in the coming weeks.

Peerawat Saengthong??? from the Internal Security Operations Command, which is chaired by Thai Prime Minister Prayuth Chan-ocha, said the navy would provide food and water "on humanitarian grounds" to boats yet to enter Thai territorial waters.

Colonel Peerawat said if boats entered Thai waters "we will act according to the law by pushing boats away".

"If they are discovered on shore we must detain them," he said.

Thailand has been criticised in the past for pushing boatloads of refugees back out to sea. Many refugees were then kidnapped and abused by people-smugglers.

But Thai authorities cracked down on the people-smuggling trade after the discovery in 2015 of mass graves in jungle camps where hundreds of mostly Rohingya refugees had been brutally exploited.

Investigations led to 102 people being convicted and sentenced to long jail terms, including a senior army general and 21 government officials.

The crackdown resulted in a secondary crisis in which smugglers, fearing arrest, abandoned boatloads of refugees at sea.

Mr Sunai urged Thailand to scrap the push-back policy and take the lead in setting up regional search and rescue operations to help Rohingya boats. He said Thailand should allow the UN refugee agency UNHCR unhindered access to Rohingya who reach Thailand so they can seek refugee status.

He said temporary shelters should also be provided before the refugees can be settled in third countries.

"In this regard it is critical for other governments to provide necessary assistance to Thailand."

Oliver White, UNICEF Australia senior policy adviser, who has just returned from Bangladesh, said unless life-saving assistance is delivered in a strategic and orderly manner to the Rohingya camps "the threat of death from malnutrition, diseases like cholera and measles, frustration-driven rioting and escalating levels of sex and gender-based violence will see this crisis become a catastrophe".

More than 250,000 of those who have arrived in the Bangladesh camps are children. Of these half are under five and almost all need malnutrition prevention and treatment.

Almost 55,000 pregnant and lactating women also need treatment.

"The need is simply enormous," Mr White said.

"We are talking about massive numbers of people who are arriving starving, with sick and wounded children, needing sanitation and shelter on a huge scale."

Filippo Grandi, the UN High Commissioner for Refugees said on Tuesday, after visiting the camps, the situation remained desperate.

"We risk a dramatic deterioration if aid is not rapidly stepped up," he said.