Thailand claimed that Cambodia fired heavy weaponry into its territory on Friday as talks on a possible ceasefire between the two countries continued.

The Cambodian action early Friday “forced the Thai side to take necessary self-defense measures,” according to a statement published at a daily press briefing. One Thai soldier was also “severely injured” on Thursday after stepping on what Thailand said was a newly laid landmine.

Both Thailand and Cambodia have blamed each other for starting the aggression and neither accepts claims made by the other side. Talks between the two nations began on Wednesday in Thailand’s Chanthaburi province, which borders Cambodia, and are scheduled to take place until Saturday.

U.S. Secretary of State Marco Rubio has called on Thailand and Cambodia to return to the U.S.-brokered ceasefire agreement signed in October. He said last week that the U.S. is “cautiously optimistic” about getting the two sides back to compliance.

The worst fighting in recent years stems from a decades-long standoff along the rugged border that has occasionally broken out into clashes. But with both governments now tapping into rising nationalist sentiment at home, there’s less incentive for de-escalation, raising the risk of a wider, more prolonged confrontation, according to analysts.

Thailand has also tightened oil shipments to Cambodia to limit its neighbor’s ability to fight.

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