The US has asked Thailand and Cambodia to "cease hostilities immediately" as border clashes extended for a third day, killing at least 10 people and displacing hundreds of thousands.

The two nations must follow de-escalatory measures outlined in a peace accord brokered by US President Donald Trump in October, Secretary of State Marco Rubio said.

Trump has also said that he would "make a phone call" to stop the fighting, which is the most serious escalation since clashes in July killed dozens of people.

Both countries have blamed each other for re-igniting the fighting, which has seen air strikes and exchanges of artillery fire.

Thailand's defence ministry said Wednesday military actions were "limited in scope and employed as a last option".

"Peace must come with the safety and security of our citizens, full stop," the ministry's spokesman said.

Cambodia on the other hand accused Thailand of launching "aggressive military attacks" that targeted civilian institutions and "sacred cultural sites", including historic temples along the disputed border.

Also on Wednesday, Cambodia announced it was pulling out from the South East Asian Games that is being hosted in Thailand.

The Cambodian National Olympic Committee cited "serious concerns and requests" from the families of its athletes for the withdrawal. It added that the decision was "not made lightly".

United Nations' Secretary-General António Guterres urged both sides to "exercise restraint and avoid further escalation", noting how their dispute has led to "significant civilian casualties, damage to civilian infrastructure, and displacement on both sides".

The century-old border dispute between the South East Asian neighbours dramatically escalated on 24 July with a Cambodian rocket barrage into Thailand, followed by Thai air strikes.

That set off five days of intense fighting, which left dozens of soldiers and civilians dead. Later that month, Bangkok and Phnom Penh agreed to an "immediate and unconditional ceasefire" brokered by Malaysian Prime Minister Anwar Ibrahim and Trump - who at the time threatened to stop tariff negotiations until the hostilities stopped.

In October, Trump claimed a historic achievement in ending the border conflict after both sides signed a ceasefire agreement, but tensions have continued to simmer.

Violence this week has expanded into at least six provinces in north-eastern Thailand and five provinces in Cambodia's north and north-west.

Thailand and Cambodia have been contesting territorial sovereignty along their 800km land border for more than a century, since the borders of the two nations were drawn after the French occupation of Cambodia.

This week, several countries, including the UK, US and Japan, have issued warnings against travelling to the border areas citing the renewed fighting.