Cambodia has charged two Thai nationals with espionage, in a move that could reignite a diplomatic row.
They were among seven Thai politicians and activists charged with illegal entry after crossing into a disputed border area in December.
Cambodian officials cited "new evidence" for bringing the additional spy charges against two of the men.
Ties between Cambodia and Thailand, fractured by a border dispute, have only recently been repaired.
"According to new evidence the authorities have gathered, we also charged two of them with attempting to gather information which affects national defence," Cambodian prosecutor Sok Roeun said.
Espionage carries a maximum sentence of 10 years in Cambodia.
Thailand's Prime Minister Abhisit Vejjajiva has insisted the men "meant no harm", and the foreign ministry is seeking their release on bail.
The Thai group were arrested on 29 December and charged with illegal entry and trespassing on a restricted military zone in Cambodia.
Analysts are watching the current case closely, as several of those detained are members of the "yellow-shirt" People's Alliance for Democracy.
This group has made nationalism, and in particular border disputes with Cambodia, a key part of its political platform.
Yellow-shirt activist Veera Somkwamkid has been charged with espionage, as well as his secretary Ratree Pipatanapaiboon, the Bangkok Post reported.
Another of those detained for trespassing is a member of parliament from Thailand's governing Democrat Party, Panich Vikitsreth.
The border issue is a flashpoint for the two neighbours. In recent years there have been sporadic exchanges of fire in areas around a disputed hill-top temple that both sides claim.
Cambodia's Prime Minister Hun Sen had also angered the Thai government by giving an advisory role to ousted Thai leader Thaksin Shinawatra.
But three Thais held for trespassing were recently returned to Thailand without fuss.