A truce agreement between Thailand and Cambodia came into effect in the early hours of Tuesday, testing whether it will halt five days of open combat that have engulfed their jungle-clad frontier.

Both sides agreed an "unconditional" ceasefire would start at midnight on Monday to end battling over a smattering of ancient temples in disputed zones along their 800-km border.

Jets, rockets and artillery have killed at least 38 people since Thursday and displaced nearly 300,000 more — prompting intervention from US President Donald Trump over the weekend.

The flare-up was the deadliest since violence raged sporadically from 2008-2011 over the territory, claimed by both because of a vague demarcation made by Cambodia's French colonial administrators in 1907.

The peace deal is set to see military commanders from both sides meet at 7am, before a cross-border committee is convened in Cambodia to further salve tensions on August 4.

Both sides are courting Trump for trade deals to avert his threat of eye-watering tariffs, and the US State Department said its officials had been "on the ground" to shepherd peace talks.

The joint statement said China also had "active participation" in the talks, hosted by Malaysian Prime Minister and Asean bloc chair Anwar Ibrahim in his country's administrative capital Putrajaya.