SEOUL, April 27 (Reuters) - Some 400 North Korean refugees ended their hunger strike at a Thai immigration detention centre after receiving a promise to fly 25 of them this week to South Korea for asylum, a humanitarian worker said on Friday.

Thai authorities said on Thursday that the strike was over after they reached a certain agreement with South Korea, without elaborating. The refugees had refused food since Tuesday to protest against processing delays and harsh conditions.

"They ended the hunger strike early Thursday night," a Seoul-based humanitarian worker, Peter Jung, of Justice for North Korea said by telephone.

South Korea's foreign ministry, which typically refrains from speaking on North Korean refugees, declined to comment.

North Korean refugees are almost always granted citizenship after arriving in South Korea.

Ten of them will be flown to the South on Friday, followed by another 15 on Saturday, Jung said, adding that Thai and South Korean authorities had given assurances of safe passage.

The processing will then slow to about 20 each month under the agreement, he said.

"But that is not nearly fast enough. They are dying a slow death inside."

Jung and a U.N. worker have said the refugees, most of whom are women, are crammed into a space for a far smaller number.

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