Fighting between Cambodia and Thailand escalated along their contested border on Tuesday, according to Reuters, as the Southeast Asian neighbors both said they would not back down in defending their sovereignty.

With each side blaming the other for starting Monday's renewed clashes, it was unclear how or if a fragile ceasefire brokered by U.S. President Donald Trump in July could be salvaged.

Cambodia's influential former leader Hun Sen said his country waited 24 hours to honor the ceasefire and allow for evacuations before launching counterattacks overnight against Thai forces.

"Cambodia needs peace, but Cambodia is compelled to counterattack to defend our territory," he said in a Facebook post, saying strong bunkers and weapons gave Cambodian forces the advantage in defending against an "invading enemy".

In Thailand, military officials said there were clashes in five border provinces, and a Navy-led operation in its Trat Province to expel Cambodian soldiers was expected to end soon. They said Cambodia was using artillery, rocket launchers, and bomb-dropping drones to attack Thai forces.

"Thailand is determined to defend its sovereignty and territorial integrity, and therefore military measures must be taken as necessary," Defense Ministry spokesperson Rear Admiral Surasant Kongsiri told the media briefing, where other military officials also spoke.

Cambodia's Defense Ministry accused Thailand of "brutal and unlawful actions", saying nine civilians were killed since Monday and 20 seriously injured. Thai officials said three soldiers had died in the fighting and 29 people had been injured.