Long-festering tensions over border territory have escalated into armed conflict between Cambodia and Thailand, leading to dozens of deaths on both sides and displacing tens of thousands of people.

Thailand and Cambodia are engaged in their worst fighting in over a decade, exchanging heavy artillery fire across their disputed border, with at least 16 people killed and tens of thousands displaced.

Thailand and Cambodia exchanged fire along their disputed border for a second day on Friday, officials from both countries said, with more than a dozen people reported killed and more than 135,000 civilians evacuated.

Thailand has one of the best-equipped and trained air forces in Southeast Asia, with an estimated 46,000 personnel, 112 combat-capable aircraft, including 28 F-16s and 11 Swedish Gripen fighter jets, and dozens of helicopters.

Thailand favours bilateral negotiation rather than third-party mediation to resolve its military conflict with Cambodia, two Thai officials said, as fighting along their disputed border continued unabated.

"The U.S. already flunked the test and that should be a wakeup call," a former senior U.S. State Department official told Newsweek.

Thailand has rejected third-party mediation offers from the US, China, and Malaysia to end escalating border clashes with Cambodia, insisting on bilateral talks.

Thailand’s military conflict with Cambodia is inflaming nationalist tensions at home and threatening embattled leader Paetongtarn Shinawatra, who had already been suspended as prime minister for her handling of the border dispute.