Thailand, Indonesia and Malaysia should fulfill their obligations under international maritime law by rescuing thousands of migrants adrift at sea and avoiding "mass casualties," shipping experts said on Friday.

Thousands of mostly Rohingya Muslims fleeing persecution in Myanmar and others escaping poverty in Bangladesh are stranded on boats as governments in the region seek to prevent them from landing, despite a request by the United Nations to rescue them.

"We will have mass casualties on our hands if there is not an immediate and concerted search-and-rescue operation by countries in the region," said David Hammond, a maritime law expert and founder of charity Human Rights At Sea.

The migrants have been at sea in rickety boats for weeks with little water and food following a crackdown by the Thai government on human trafficking.

"Turning these vulnerable people away because of political concerns that they may wish to seek refugee status is unacceptable and violates the obligation of governments to help people in distress at sea," Hammond told the Thomson Reuters Foundation.

The International Chamber of Shipping (ICS), which represents the global shipping industry, said merchant ships as well as governments have a humanitarian obligation enshrined in international maritime law and conventions to help people in distress at sea.

"It's a well-honored maritime tradition for ships to rescue anyone in distress at sea but coastal states also have an obligation to come to the rescue, and we expect them to honor this, including taking migrants ashore," ICS spokesman Simon Bennett told the Thomson Reuters Foundation.

As members of the International Maritime Organization (IMO), Thailand, Indonesia and Malaysia are obliged to work together so those rescued reach a place of safety as soon as possible.

But the exact terms of such obligations are a grey area because they do not clearly define what governments should do to help migrants adrift in the sea.

"The regulations are worded very carefully in order not to be too prescriptive," a spokeswoman for the IMO, an agency of the United Nations, told the Thomson Reuters Foundation.

"They state that there should be cooperation and coordination but without specifying precisely what needs to be done. That ... does mean it's open to interpretation."

However, Hammond said the case was clear.

"Someone who is at sea, has no food and has put people at notice that they are in trouble within the sight of land, I would say there is a lawful obligation for governments to help them," he said.

Merchant ships could boost search-and-rescue operations to help boat people in the Andaman Sea and the Strait of Malacca, one of the world's busiest shipping channels, Hammond said.

Last year, merchant ships were deployed to rescue more than 40,000 out of a total of more than 200,000 migrants attempting to cross the Mediterranean Sea from Africa and the Middle East, according to the ICS.

The U.N. and the United States have asked for a regional response to avoid a humanitarian disaster but so far none of the countries involved in the crisis has responded.

U.N. Secretary General Ban Ki-moon expressed alarm on Thursday after Thailand declined permission for a migrant boat to land while Malaysia said it would push boats back out to sea.

Nearly 800 "boat people" were brought ashore in Indonesia on Friday by fishermen but other vessels were sent back to sea.

Thailand, which will host a regional meeting on the issue on May 29, said on Friday the migrants found on the boat near its southern coast in the Andaman Sea had not wanted to come to Thailand but had wanted to go to a third country.

The authorities fixed the boat's engine and handed over food, water and medicine before towing it back out to sea.