A long-simmering border dispute between Thailand and Cambodia erupted into armed clashes Thursday that have killed at least 12 people on the Thai side of the frontier.

Fighting has resumed for a second day between Cambodia and Thailand along their disputed border, a Thai Army spokesman told CNN on Friday.

Thai and Cambodian soldiers fired at each other in contested border area Thursday after the nations downgraded their diplomatic relations in a rapidly escalating dispute. It was not immediately clear if the clash was ongoing.