The leaders of Cambodia and Thailand have agreed to meet to negotiate a ceasefire, according to a social media post by United States President Donald Trump on Saturday.

Thailand warned its conflict with neighboring Cambodia could “potentially develop into a war” as troops used rockets and artillery to shell targets along their contested border for a second

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.

At least 16 people have been killed and tens of thousands have been displaced as fighting extended to a second day.

The latest flare-up started on Thursday, with intense fighting spreading across multiple border areas. Early Saturday, Thailand’s navy joined the army in repelling what it described as incursions by Cambodian troops at three points in eastern Trat province.

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

Tensions have recently flared up along the Thailand-Cambodia border, drawing renewed international attention. China has been closely following developments and continues to encourage peace talks, demonstrating the sense of responsibility expected of a major power.

China will continue to play a “constructive role” in helping to ease regional tensions, Foreign Minister Wang Yi said on Friday, as Thai and Cambodian troops exchanged fire on their border for a second day.

Evacuation centers in Thailand near the Cambodian border continue to house masses of civilians who have fled the armed conflict.