BANGKOK (AP) — Thailand threatened Monday to suspend a U.S.-brokered ceasefire with Cambodia after a land mine explosion along their volatile border injured four Thai soldiers.

Thailand and Cambodia signed the truce agreement last month after territorial disputes between the Southeast Asian neighbors led to five days of combat in late July that killed dozens of soldiers and civilians.

Tensions have simmered and other land mine explosions before and after the clashes have also caused injuries.

The Royal Thai Army said late Monday the explosion earlier in the day injured four soldiers. A sergeant lost his right foot after stepping on a land mine while on patrol along the border in Sisaket province, while the other three suffered minor injuries from shrapnel or the impact of the blast.

It also said Thai troops regularly patrol the area and accused Cambodia of laying new land mines in violation of the truce.

The Cambodian Ministry of Foreign Affairs denied the allegations, saying most mines along the border are old mines that have not been cleared. “Cambodia remains committed to implementing the Joint Declaration, which was signed amidst much applaud from the international community,” it said.

Thai Prime Minister Anutin Charnvirakul said Monday’s land mine explosion showed that “the hostility towards our national security has not decreased as we thought it would,” adding that all actions to be carried out under the truce agreement will be halted until Thailand's demands are met. He did not elaborate on Thailand's demands.

Under the terms of the agreement, Thailand is to release 18 Cambodian soldiers held prisoner and both sides are to begin removing heavy weapons and land mines from the border area.

Thai Defense Minister Natthaphon Narkphanit said the army is still investigating whether the mine was old or was newly laid. He added that Thailand will postpone the release of the captured Cambodian soldiers, initially scheduled for later this week.

The two sides said there has been some progress on arms removal, but Thailand accuses Cambodia of obstructing the mine clearance process. Cambodia says it's committed to all terms in the agreement, including demining, and urged Thailand to release its soldiers as soon as possible.

The ceasefire was initially mediated by Malaysia, and U.S. President Donald Trump later threatened to withhold trade privileges unless both Cambodia and Thailand agreed on a truce. The deal was signed during the Association of Southeast Asian Nations summit in October.