A spokeswoman for the Cambodian defence ministry, Maly Socheata, said their troops “exercised their right to defend their sovereignty and territorial integrity against the aggression of the Thai troops.
Socheata said Thailand “violated the territorial integrity of Cambodia”.
Parts of two Cambodian provinces had been shelled by the Thai military, former leader Hun Sen said on Facebook, while also urging people not to panic.
“Cambodia has always maintained a position of wanting to resolve issues peacefully, but in this case we have no choice but to respond with armed force against armed aggression,” Prime Minister Hun Manet said on Facebook.
Thailand’s acting premier Phumtham Wechayachai said that its dispute with Cambodia remains “delicate” and must be addressed with care, and in line with international law.
Thursday’s clash comes a day after Thailand recalled its ambassador to Cambodia, following a landmine explosion that injured a Thai soldier along the border.
On Wednesday, Bangkok also said it would expel Cambodia’s ambassador.
Bilateral relations between the two countries are at their worst in more than a decade, after armed clashes in May left one Cambodian soldier dead.
In the past two months, both countries have imposed tit-for-tat restrictions and strengthened troops presence along the border.
Additional reporting by May Titthara in Phnom Penh