Cambodia has retaliated in a reignited border conflict with Thailand, the former leader of Cambodia said on Tuesday, after Phnom Penh accused Thai forces of shelling positions overnight in fighting that has killed seven civilians and a Thai soldier.

Thailand, for its part, said it was taking action to expel Cambodian forces from its territory on Tuesday

The Thai military fired shells into the border province of Banteay Meanchey after midnight, killing two people travelling on National Road 56, the Cambodian defence ministry said in a Facebook post.

Ministry spokeswoman Maly Socheata later said seven civilians had been killed and 20 wounded in Thai attacks as of Tuesday morning.

The ministry said in a separate statement that the Thai army had resumed attacks around 5am on Tuesday in border regions, including in the area of centuries-old temples, such as the Unesco world heritage site, the Preah Vihear temple.

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 military operations were launched to expel them.

Cambodian forces were increasing their presence, deploying snipers and heavy weapons, improving fortified positions and digging trenches, it said, adding that it saw the actions "as a direct and serious threat to Thailand's sovereignty".

Five days of combat in July between the two Southeast Asian nations killed dozens of people and displaced around 300,000 on both sides of the border before a truce took effect.

The two countries have blamed each other for the renewed fighting, which saw Thailand launch air strikes and use tanks against its neighbour on Monday.

Cambodia's influential former leader Hun Sen said on Tuesday his country had retaliated against Thailand, after Phnom Penh denied firing back for two days.

"After being patient for more than 24 hours in order to respect the ceasefire and for time to evacuate people to safety, yesterday evening we retaliated with more [responses] last night and this morning," the Senate president and former prime minister said in a Facebook post. "Now we fight in order to defend ourselves again."

Tens of thousands of people have evacuated from border regions since the fresh fighting began on Sunday, officials have said. (AFP/Reuters)