At least 30 people have been killed and more than 130,000 people displaced in the worst fighting between the Southeast Asian neighbours in 13 years, with both sides saying they have acted in self-defence.
The Thai navy said there were clashes in the coastal province of Trat early on Saturday, a new front more than 100km from other conflict points along the long-contested border.
The two countries have faced off since the killing of a Cambodian soldier late in May during a brief skirmish. Troops on both sides of the border were reinforced amid a full-blown diplomatic crisis that brought Thailand's fragile coalition government to the brink of collapse.
Thailand's death toll remained at 19 on Saturday, while Cambodian Defence Ministry spokesman Maly Socheata said five soldiers and eight civilians had been killed in the fighting.
In the Kanthralak district of Thailand's Sisaket province, on the border near some of the clashes, hotel worker Chianuwat Thalalai said the town had emptied.
"Nearly everybody's gone. It's almost a deserted city," the 31-year-old told Reuters.
"My hotel is still open for some of those nearer to the border area that needs a place to stay."
Thailand's ambassador to the United Nations told a Security Council meeting on Friday that soldiers had been injured by newly planted land mines in Thai territory on two occasions since mid-July - claims Cambodia has strongly denied - and said Cambodia had then launched attacks on Thursday morning.
"Thailand urges Cambodia to immediately cease all hostilities and acts of aggression, and resume dialogue in good faith," Cherdchai Chaivaivid told the council in remarks released to the media.
Cambodia's defence ministry said Thailand had launched "a deliberate, unprovoked, and unlawful military attack" on Thursday, and was now mobilising troops and military equipment on the border.
"These deliberate military preparations reveal Thailand's intent to expand its aggression and further violate Cambodia's sovereignty," the ministry said in a statement on Saturday.
Cambodia called for the international community to "condemn Thailand's aggression in the strongest terms" and to prevent an expansion of Thailand's military activities.
Bangkok reiterated it wanted to resolve the dispute bilaterally, telling the Security Council it was "deeply regrettable that Cambodia has deliberately avoided meaningful dialogue and instead sought to internationalise the issue to serve its own political objectives".
Thailand and Cambodia have bickered for decades over the jurisdiction of various un-demarcated points along their 817km land border, with ownership of the ancient Hindu temples Ta Moan Thom and the 11th century Preah Vihear central to the disputes.
Preah Vihear was awarded to Cambodia by the International Court of Justice in 1962, but tension escalated in 2008 after Cambodia attempted to list it as a UNESCO World Heritage site.
That led to skirmishes across several years and at least a dozen deaths.
Cambodia in June said it had asked the court to resolve its disputes with Thailand, which says it has never recognised the court's jurisdiction and prefers a bilateral approach.
