Five days of intense clashes between the Southeast Asian neighbours that began last week killed at least 43 people, many of them civilians, and displaced more than 300,000, until a truce brokered in Malaysia on Monday halted the fighting.
Thailand has since accused Cambodian troops of violating the ceasefire multiple times, a charge denied by authorities in Phnom Penh, who instead allege that the Thai military has wrongfully detained a number of its soldiers.
"We appeal to the Thai side to promptly return all 20 of our forces, including other forces if any are under Thai control," Cambodian Prime Minister Hun Manet said on Thursday.
In a statement, senior Cambodian defence official Lieutenant General Rath Dararoth said one Cambodian soldier had died in Thai custody since the ceasefire and his body had been returned.
He did not provide further details.
Thailand had custody of 20 Cambodian soldiers who had surrendered, including two who are under medical treatment, Thai Rear Admiral Surasant Kongsiri told reporters.
"We are investigating them to verify the facts. After this is finished, they will be released," Thailand's Acting Prime Minister Phumtham Wechayachai said, stressing the Thai military had not violated the ceasefire agreement.
As per talks between military commanders held after Monday's truce announcement, Thailand and Cambodia agreed to facilitate the return of wounded soldiers and bodies of those deceased, besides refraining from reinforcing troops along the border.
Cambodia took military attaches and diplomats to a border checkpoint on Wednesday to verify the ceasefire as both sides exchanged accusations of violating the truce.
For decades, Thailand and Cambodia have wrangled over undemarcated points along their 800km land border, with ownership of the ancient Hindu temples Ta Moan Thom and the 11th century Preah Vihear central to the disputes.
The recent truce followed a push by Malaysia and calls by US President Donald Trump's phone calls to leaders of Thailand and Cambodia, warning them that trade deals would not be concluded if the fighting continued.
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