Cambodia has withdrawn all its athletes from the Southeast Asian Games in Thailand as a surge in border clashes with its neighbor escalates into a humanitarian and security crisis, just a day after the Games officially opened.

The National Olympic Committee of Cambodia (NOCC) cited urgent safety concerns and repeated requests from athletes’ families as the primary reason for the sudden pullout.

NOCC chief Vath Chamroeun wrote to the Southeast Asian Games Federation that “due to serious concerns and requests from families, we must withdraw our delegation and arrange for their prompt return to Cambodia.”

The Games, hosted in Bangkok and the nearby coastal province of Chonburi through Dec. 20, had brought together thousands of athletes from Southeast Asia, competing in football, skateboarding, fencing, sailing, combat sports, and more.

Tuesday’s opening ceremony at the Rajamangala National Stadium, attended by Thai royals and K-pop star BamBam, had a small Cambodian delegation marching in the parade amid heightened security.