Thailand, Malaysia and Indonesia will continue to send boats holding thousands of migrants back to sea, a senior Thai official said on Wednesday, despite a UN appeal for a rapid rescue operation to avoid a humanitarian crisis.

Several thousand migrants, many of them hungry and sick, are adrift in Southeast Asian seas in boats that have been abandoned by smugglers following a Thai government crackdown on human trafficking, the United Nations has said.

"Indonesia, Malaysia and Thailand have decided not to receive boat people, as far as I am aware," said Major General Werachon Sukhondhapatipak, spokesman for Thailand's ruling junta.