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.

President Donald Trump said Saturday that Cambodia’s prime minister and the acting prime minister of Thailand had agreed to meet immediately and quickly work out a ceasefire as he sought to end the conflict between the two Southeast Asian neighbors,

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.

Thailand has launched airstrikes against Cambodian military targets along their disputed border, in an escalation of their deadly border dispute. Follow for live updates

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.

China has blamed “Western colonialists” for ongoing clashes on the Thai-Cambodian border that have left at least 15 people dead and forced tens of thousands to flee their homes.

Thai men's top seed Kunlavut Vitidsarn swept aside Malaysian Leong Jun Hao with a 23-21 21-7 win to march into the quarter-finals of the China Open on Thursday, while women's top seed An Se-young beat Sim Yu-jin 21-13 21-15 in an all-South Korean clash.