Prime Minister Hun Manet said one Cambodian died and three others were injured during Wednesday’s gunfire in Banteay Meanchey province.
He said in a statement his country remains committed to resolving border issues peacefully under the Oct 26 peace accord, but strongly condemns “the unprovoked use of violence”.
Thailand rejected the claim and accused Cambodia of firing first.
Nikorndej Balankura, a spokesman for the Thai foreign affairs ministry, claimed Cambodia used “civilians as human shields”.
The incident marks the most serious escalation since a July ceasefire ended five days of deadly clashes.
It came just days after Thai Prime Minister Anutin Charnvirakul said his country would halt the peace process following a land-mine blast that injured Thai soldiers near the border earlier this week.
Thailand said the suspension would remain in effect until hostilities cease and that it would delay the release of 18 Cambodian soldiers under its custody.
The renewed violence raises doubts about the future of the US-backed peace deal.
In July, Trump had threatened to cancel trade talks with both nations unless they stopped fighting, and Thai officials have said that holding a peace summit-style meeting during his Asia trip last month was a condition for securing trade terms.
Earlier yesterday, Anutin said Thailand would prioritise its interests and the safety of its citizens, even at the cost of the country’s trade deal with the US.
“What about tariff negotiations? I don’t care. If we can’t sell to that country, we’ll find others,” Anutin said in a speech at Thailand’s National Defense College.
“How can we put our lives in the hand of one country?,” he added.
His comment underscores how domestic political sensitivities about the border dispute now outweigh economic and diplomatic considerations, especially ahead of an expected election early next year.
The Thai-Cambodian conflict stems from long-running disputes over colonial-era border maps and treaties.
While relations had stabilised after deadly clashes in 2011, tensions have resurfaced this year, culminating in July’s intense fighting and the latest flare-up.