Thailand said it was taking action to expel Cambodian forces from its territory on Tuesday, as renewed fighting between the two South-east Asian neighbours spread along the disputed border.

Each side has blamed the other for the clashes, which have derailed a fragile ceasefire brokered by US president Donald Trump that ended five days of fighting in July.

Cambodia’s defence ministry said two civilians had been killed overnight, taking its death toll to six. One Thai soldier has died in the fighting.

In a statement on Tuesday morning, the Thai Navy said Cambodian forces had been detected inside Thai territory in the coastal province of Trat and that military operations were launched to expel them, without providing further details.

Cambodian prime minister Hun Manet said late on Monday that Thailand “must not use military force to attack civilian villages under the pretext of reclaiming its sovereignty”.

Earlier, Cambodia said it had not retaliated even after its forces came under sustained attack.

The Thai Navy said Cambodian forces were increasing their presence, deploying snipers and heavy weapons, improving fortified positions and digging trenches, adding it saw the actions “as a direct and serious threat to Thailand’s sovereignty”.

Monday’s clashes were the fiercest since a five-day exchange of rockets and heavy artillery in July, when at least 48 people were killed and 300,000 displaced, before Trump intervened to broker a ceasefire.

Thailand evacuated 438,000 civilians across five border provinces and authorities in Cambodia said hundreds of thousands of people had been moved to safety. Thailand’s army said 18 soldiers were wounded and Cambodia’s government reported nine civilians injured.

Thailand and Cambodia have for more than a century contested sovereignty at un demarcated points along their 817km (508-mile) land border, with disputes over ancient temples stirring nationalist fervour and occasional armed flare-ups, including a deadly week-long artillery exchange in 2011.

Tensions rose in May after the killing of a Cambodian soldier during a skirmish, which led to a major troop buildup at the border and escalated into diplomatic breakdowns and armed clashes.