Soldiers from the two countries have been stationed in the area since July
Troops from Thailand and Cambodia have exchanged heavy gunfire twice during the day in a disputed border area.
The Thai army said at least one of its soldiers was killed and seven others wounded by Cambodian rocket fire.
The second more serious outbreak of firing near the ancient temple of Preah Vihear happened in the afternoon, despite both sides meeting for talks.
Each side accused the other of firing first, in what is seen by analysts as a significant heightening in tensions.
The BBC's Guy De Launey in Phnom Penh says the clashes come days before fresh talks between the two sides over the disputed territory.
The Cambodia-Thailand Joint Border Committee is expected to hold three days of talks in the Cambodian resort town of Siem Reap, starting on Sunday.
The first, brief exchange of fire took place early on Friday, in the disputed zone where opposing troops are sometimes stationed within metres of each other.
Talks over lunch between the two sides' commanders were supposed to ease the tension, says our correspondent.
TEMPLE DISPUTE TIMELINE
International court awards temple to Cambodia, but surrounding land remains undesignated
Khmer Rouge guerrillas occupy site
Thai troops block access over water row
Unesco lists temple as a World Heritage Site
Thai FM quits after court rules he violated constitution for backing Cambodia's Unesco bid
Both sides move troops to temple area
Troops exchange fire, leaving two Cambodian soldiers dead
But a Cambodian government official told the BBC that the second exchange was spread over a wider area and involved heavier weaponry than the morning incident.
One Thai soldier was killed and seven wounded, the Thai foreign ministry and the military were quoted by media as saying.
The fighting appeared to have started after a Thai patrol visited the site where a Thai soldier was injured after stepping on a landmine on Thursday.
"After talks between the two sides failed, the Cambodian side started to walk away and turned back to open fire at Thai troops with rifles and RPG rockets, forcing the Thai side to fire back in self-defence," said a statement from Thailand's foreign ministry.
"It was an accident, a misunderstanding among officials on the ground, which is common when you are closely positioned," said Thai Defence Minister Pravit Wongsuwan.
However, the Cambodian foreign ministry called it an "intended aggressive invasion by the Thai military", and said a letter of protest would be sent to Thailand.
Thailand and Cambodia share a border that runs for nearly 800km (500 miles), much of it heavily mined.
An international court awarded Preah Vihear temple to Cambodia in 1962, but a 5-sq-km (1.9-sq-mile) patch of land surrounding it remains the subject of rival territorial claims.
Soldiers from the two countries have been stationed in the area since tensions increased in July last year, after it was listed as a World Heritage Site.
Two Cambodian soldiers were killed there in October in a gun battle. Last week, Cambodia accused Thai troops of crossing over briefly into its territory.