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

Of course, trouble at the 508-mile (817 km) shared border is nothing new. For over a century, Thailand and Cambodia have contested sovereignty at various undemarcated points in the thick jungle punctuated with culturally-significant temples albeit with scant strategic or economic value.

SISAKET, Thailand/WASHINGTON, July 26 (Reuters) - U.S. President Donald Trump said on Saturday that the leaders of Cambodia and Thailand had agreed to meet immediately to quickly work out a ceasefire, as he sought to broker peace after three days of fighting along their border.

The Thai warning came as the U.S. expressed its grave concern about the most serious fighting in years between the Southeast Asian neighbors. The U.S. and China compete for influence in fast-growing Southeast Asia, and both have good relations with Thailand and Cambodia.

China has dismissed allegations that it was supplying weapons to Cambodia amid its ongoing border conflict with Thailand that has led to at least 33 people dying and hundreds of thousands displaced.

Thailand and Cambodia exchanged heavy artillery fire for a second day on Friday as border fighting intensified and spread, while Cambodia's leader said Thailand had agreed to a Malaysian ceasefire proposal but then backed down.

Thailand and Cambodia pounded each other with heavy artillery fire for a third day on Saturday, as a border conflict that killed at least 33 people and displaced more than