Kuala Lumpur: Malaysia said on Thursday it would push boats full of migrants back to sea, a policy that has drawn criticism from the U.N. refugee agency as thousands remained adrift in Southeast Asian waters.

The UNHCR has said several thousand migrants have been abandoned at sea by smugglers following a Thai crackdown on human trafficking and has warned the situation could develop into a ‘massive humanitarian crisis’.

The crackdown has made traffickers wary of landing in Thailand, the preferred destination for the region's people smuggling networks, and led to a surge in migrants to Indonesia and Malaysia.

‘We are sending them the right signal, to send them to where they came from,’ Malaysian Deputy Home Minister Wan Junaidi Tuanku Jaafar said.

‘Their country is not at war. If there is nothing wrong with the ship, they should sail back to their own country.’

Many of the arrivals are Rohingya, a stateless Muslim minority from Bangladesh and Myanmar described by the United Nations as one of the most persecuted minorities in the world.

An estimated 25,000 Bangladeshis and Rohingya boarded rickety smugglers' boats in the first three months of this year, twice as many in the same period of 2014, the UNHCR has said.

The agency has called for a coordinated search and rescue operation.

‘The first priority is to save lives,’ said Volker Turk, the UNHCR's Assistant High Commissioner for Protection, in a statement on Wednesday. ‘It is key for states to share the responsibility to disembark these people immediately.’

Malaysia said the UNHCR should find another country for the migrants.

‘I don't see why we are under pressure,’ Wan said. ‘We are doing what we think we should do. We have to consider what our people want to see us doing. They don't want to see immigrants come into our country.’

Malaysian officials could not confirm media reports of at least two more boats with hundreds on board being pushed back to sea. ‘We don't have any information,’ Abdul Aziz Yusoff, commander of the marine operations force, said.

Thailand ordered a clean-up of suspected traffickers' camps last week after 33 bodies, believed to be of migrants, were found in shallow graves near the Malaysian border.

The navies of both Thailand and Indonesia said their policy was to offer food and water to migrants on Wednesday and not to send boats back out to sea.

But Thai government officials have said the country is not receiving boatpeople - that is why they are arriving in Malaysia and Indonesia.

More than 1,600 migrants arrived Indonesia and Malaysia at the weekend. Malaysia detained many. Indonesia provided food, water and medical supplies to around 500 on a boat off the coast of the northwestern province of Aceh on Monday, before sending the vessel towards Malaysia.

‘There is some confusion on policy,’ Vivian Tan, the spokeswoman for UNHCR in Thailand, said on Thursday. ‘The numbers are quite overwhelming.’

The United States last year downgraded Thailand and Malaysia to its list of the world's worst centres of human trafficking, dumping them in the same category as North Korea and Syria.