Three Thai soldiers were injured by a landmine on Saturday while patrolling a border area near Cambodia, the Thai army said in a statement, days after the two neighbours agreed to a detailed ceasefire following a violent five-day conflict last month.

One soldier lost a foot and two others were injured after one of them stepped on a landmine as they patrolled an area between Thailand’s Sisaket and Cambodia’s Preah Vihear provinces.

“The soldiers are being treated at a hospital,” the army said.

Thailand said the incident took place within its territory and in an area recently cleared of landmines. It added that it would lodge a complaint against Cambodia for violating a treaty that bans the use of landmines and for infringing Thai sovereignty, the Thai foreign ministry said in a statement.

Both Thailand and Cambodia are signatories to the Ottawa Convention against landmines.

Cambodia’s defence ministry said on Saturday that it “has yet to receive clear confirmation from Cambodian frontline forces concerning the explosion”, adding that its army has been strictly respecting the spirit of the ceasefire.

The incident is the third time in a few weeks that Thai soldiers have been injured by mines while patrolling along the border. Two previous incidents led to the downgrading of diplomatic relations and triggered five days of violent clashes.

The Southeast Asian neighbours were engaged in deadly border clashes from July 24-28, in the worst fighting between the two in more than a decade. The exchanges of artillery fire and jet fighter sorties claimed at least 43 lives and left over 300,000 people displaced on both sides.

A fragile ceasefire has been holding since Thailand and Cambodia agreed on Thursday to allow observers from the Association of Southeast Asian Nations to inspect disputed border areas to ensure hostilities do not resume.

Bangkok earlier accused Cambodia of planting landmines on the Thai side of the disputed border that injured soldiers on July 16 and July 23. Phnom Penh denied it had placed any new mines and said the soldiers had veered off agreed routes and triggered old landmines left from its decades of war.