Thailand has accused Cambodia of "deliberately" violating a ceasefire the two countries agreed on Monday to end border clashes that have killed at least 33 people and displaced thousands.

It's a shaky start for the ceasefire, which is aimed at bringing an end to five days of bombardment and rocket attacks along their shared border.

The Thai military says it stopped firing after midnight, but that it continued to receive gunfire from the Cambodian side "at multiple locations" up until this morning.

But Cambodia's defence ministry told the AFP news agency that there had been "no armed clashes" between the two sides since the ceasefire began at midnight.