The nations agreed a ceasefire starting Tuesday after five days of clashes killed at least 43 on both sides, as a long-standing dispute over contested border temples boiled over into open combat on their 800-kilometre (500-mile) frontier.
But Thailand's foreign ministry said its troops in Sisaket province "came under attack by small arms fire and grenade assaults launched by Cambodian forces" in an offensive which continued until Wednesday morning.
"This represents a flagrant violation of the ceasefire agreement," said a foreign ministry statement.
Thai government spokesman Jirayu Huangsab also reported overnight clashes but said in a statement "the Thai side maintained control of the situation" and "general conditions along the border are reported to be normal" from 8:00 am (0100 GMT).
Cambodia has previously denied breaking the truce, intended to end fighting which has seen the two countries evacuate a total of more than 300,000 people from the border region.
At a temple in Thailand's Surin city serving as a shelter and field kitchen 50 kilometres away from the frontier, volunteer Thanin Kittiworranun said evacuees remain in limbo.
"We don't believe Cambodia will hold the ceasefire," the 65-year-old told AFP.
But an AFP journalist on the Cambodian side who heard a steady drumbeat of artillery fire since fighting began last Thursday reported hearing no blasts between the start of the truce and Wednesday morning.
And Beijing said deputy foreign minister Sun Weidong hosted a Shanghai meeting with Thai and Cambodian officials where both "reaffirmed to China their commitment to abide by the ceasefire consensus".
- 'Still fragile' -
The armistice got off to a shaky start in the early hours of Tuesday, with Thailand accusing Cambodia of continuing attacks in "a clear attempt to undermine mutual trust" -- before peace generally prevailed.
Meetings between rival commanders along the border -- scheduled as part of the pact -- went ahead, with Thailand's army saying de-escalation steps were agreed including "a halt on troop reinforcements or movements that could lead to misunderstandings".
But later in the day a foreign affairs spokeswoman for Bangkok's border crisis centre, Maratee Nalita Andamo, warned: "In this moment, in the early days of the ceasefire, the situation is still fragile".
Jets, rockets and artillery have killed at least 15 Thai troops and 15 Thai civilians, while Cambodia has confirmed only eight civilian and five military deaths.
The flare-up has surpassed the death toll of 28 in violence that raged sporadically from 2008 to 2011 over the territory, claimed by both nations because of a vague demarcation made by Cambodia's French colonial administrators in 1907.
The peace pact was sealed in Malaysia after intervention from US President Donald Trump -- who both Thailand and Cambodia are courting for a trade deal to avert his threat of eye-watering tariffs.
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