Of those who did not reach those countries, many migrants from Myanmar — members of the Rohingya ethnic group who had been fleeing persecution — may have returned there.

Rohingya in Myanmar have told Ms. Lewa’s group that as many as 2,000 people who had been held in boats off the Myanmar coast have now gone back ashore, paying the smugglers as much as $300 to smuggle them back in so that they could avoid being arrested for illegally re-entering Myanmar.

The Myanmar government does not consider them citizens, rendering them effectively stateless.

One Rohingya shopkeeper at a camp for displaced people in Sittwe, a coastal city that is the capital of Rakhine State, said he had counted about 50 neighbors returning home from ships in recent days. The shopkeeper, U Maung Maung Tin, said they decided to return after it became clear that their route to Malaysia had been cut off.

He said they had to pay the equivalent of $180 to $275 to the smugglers before being allowed to disembark. The smugglers sometimes kept people in boats just offshore for months while they filled their holds with as many people as possible before setting sail some 600 miles across the Andaman Sea to southern Thailand.

The Myanmar government seized a boat with 208 people on board last week. Bangladesh has agreed to accept 200 of them, whom the Myanmar government said were Bangladeshis, the Myanmar government reported on Tuesday.