Brigadier Bun Thean said the two sides had exchanged gunfire, after reports that 500 Thai soldiers were massing on their side of the border.
“Cambodian and Thai troops are opening fire at each other now,” he told AFP from the scene, accusing Thailand of firing the first shot.
“Thai troops entered our territory … It is heavy gunfire.”
A senior Thai military official who did not wish to be named confirmed that fighting had broken out near the border, but insisted that it was isolated.
“It’s small-scale fighting and the Thai authorities are trying to contain the incident,” he told AFP in Bangkok.
The fighting broke out in a small patch of disputed land about two kilometres (1.2 miles) from the 11th century Khmer temple, an UN World Heritage site which is on Cambodian territory.
Cambodia and Thailand have been sparring over land near the temple since July, and tensions soared this week after about 80 Thai troops entered a disputed area, enraging Cambodia.
Thai television on Wednesday showed military trucks loaded with tanks heading toward the border and troops setting up mortars, while the Thai air force spokesman said their fighter jets were on stand-by.
“Normally we have fighter jets on stand-by at various regional headquarters ready for operation within five minutes, but under the current circumstances we have increased our readiness,” Group Captain Montol Suchookorn told AFP.
He said transport planes were also being prepared in case they were needed to evacuate Thai nationals from Cambodia, but added that he expected the stand-off to be settled through talks, rather than an armed conflict.
“I do hope the use of force is the last resort,” he said, before the reports of gunfire.
Colonel Sunsern Kaewkumnerd, the Thai army spokesman, earlier confirmed that there had been a reinforcement of troops by both Thailand and Cambodia, but declined to say how many Thai soldiers were being readied.
In Phnom Penh, General Neang Phat, a secretary of state at the defence ministry, refused to reveal the number of Cambodian troops stationed near the border, but said they were ready to protect their territory.
“We have enough troops to protect our territorial integrity,” he told AFP.
Tensions between the neighbours flared again this week after failed talks on Monday aimed at cooling a months-long stand off over land near Preah Vihear.
Cambodian Prime Minister Hun Sen then issued an ultimatum to the Thai side after he accused more than 80 Thai soldiers of entering one of a handful of disputed areas — leave or risk conflict.
Cambodian officials insisted that the troops did leave before the Tuesday mid-day deadline, but Thailand’s foreign minister said they were standing their ground.
The stand-off between the neighbours first flared in July after Preah Vihear was awarded World Heritage status by the UN cultural body UNESCO, angering some Thai nationalists who still claim ownership of the site.
The situation quickly escalated into a military confrontation, with up to 1,000 Cambodian and Thai troops facing off for six weeks, although both sides in August agreed to reduce troop numbers in the disputed area.