Following peace talks in Malaysia on Monday, both sides agreed an unconditional ceasefire would start at midnight to end fighting over a smattering of ancient temples in disputed zones along their 800-kilometer border.

"At the time the agreement took effect, the Thai side detected that Cambodian forces had launched armed attacks into several areas within Thai territory," said Thai army spokesman Winthai Suwaree.

"This constitutes a deliberate violation of the agreement and a clear attempt to undermine mutual trust, he added in a statement. "Thailand is compelled to respond appropriately, exercising its legitimate right to self-defense."

Cambodia's defense ministry spokeswoman Maly Socheata insisted there had been "no armed clashes against each other in any regions".

However, both sides said morning meetings scheduled between opposing regional commanders along the border as part of the truce pact had either begun or were still scheduled to start.

"I saw photos of the two leaders shaking hands," said 32-year-old pharmacy worker Kittisak Sukwilai in the Thai city of Surin -- 50 km from the border.

"I just hope it's not just a photo op with fake smiles -- and that those hands aren't actually preparing to stab each other in the back."

In Cambodia's Samraong city -- 20 km from the frontier -- an AFP journalist said the sound of blasts stopped in the 30 minutes leading up to midnight, with the lull continuing until mid morning.

"The frontline has eased since the ceasefire at 12 midnight," Cambodian Prime Minister Hun Manet said in a Tuesday morning message on Facebook.

'Good faith'

Jets, rockets and artillery have killed at least 38 people since last 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.

"When I heard the news I was so happy because I miss my home and my belongings that I left behind," Phean Neth told AFP on Monday evening at a sprawling camp for Cambodian evacuees on a temple site away from the fighting.

"I am so happy that I can't describe it," said the 45-year-old.

A joint statement from both countries -- as well as Malaysia which hosted the peace talks -- said the ceasefire was "a vital first step towards de-escalation and the restoration of peace and security".

A spokesman for UN Secretary General Antonio Guterres said in a statement Monday evening that "he urges both countries to respect the agreement fully and to create an environment conducive to addressing long-standing issues and achieving lasting peace".

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.

Hun Manet thanked Trump for his "decisive" support, while his counterpart, Thai acting Prime Minister Phumtham Wechayachai, said it should be "carried out in good faith by both sides".

Post-truce tariff deals

Thai King Maha Vajiralongkorn marked his 73^rd birthday on Monday, but a notice in the country's Royal Gazette said public celebrations scheduled for Bangkok's Grand Palace had been cancelled amid the strife. 

Each side had already agreed to a truce in principle, while accusing the other of undermining peace efforts and trading allegations about the use of cluster bombs and targeting of hospitals.

Thailand says 11 of its soldiers and 14 civilians have been killed, while Cambodia has confirmed only eight civilian and five military deaths.

More than 138,000 people have fled Thailand's border regions, while around 140,000 have been driven from their homes in Cambodia.