Defence Minister Phumtham Wechayachai said four Thai fishing crew members were not released by Myanmar on Saturday as earlier expected because their release process is still underway.
Mr Phumtham said on Sunday that the release of the four Thais was being processed and the Foreign Affairs Ministry was following up on the issue closely.
Relatives had already met the four Thai detainees in Kaw Thaung and found that all of them were in good physical and mental health, Mr Phumtham said.
The minister confirmed that they would be released soon but he could not specify an exact date because of procedural issues regarding the release.
According to a recent report, the crew members were sentenced to four to six years in a Myanmar jail after their alleged intrusion into the Myanmar waters late November.
Earlier Mr Phumtham had predicted their release on Jan 4 (Saturday), Myanmar's Independence Day.
On Saturday 151 other Thai prisoners were released from Myanmar.
Nikorndej Balankura, spokesman for the Foreign Affairs Ministry, expressed disappointment over the prolonged detention of four Thai fishing crew members in Myanmar.
He said on Sunday that it was disappointing that the four Thais were not freed on Saturday and the ministry had done its best to obtain their release and would continue to do so.
The spokesman also said that the Myanmar Foreign Affairs Ministry had been cooperative and had tried to push for the quick release of the Thai fishermen. However, the release also depended on internal processes in Myanmar, he said.
He said that officials from the Thai embassy in Yangon received permission to visit the four Thais in Kawthaung and found that they were in good physical and mental health and received three meals a day.
Mr Nikorndej said that the case was sensitive when it came to the release of the Thai fishermen, because it involved issues concerning the fishing industries of both sides and overall bilateral ties.
Patience and careful negotiation were essential for the matter, he said.