Thailand's army has accused Cambodia of breaching a newly-signed ceasefire deal reached after weeks of deadly clashes that forced nearly one million people from their homes.

In a statement, the Thai army said than more than 250 unmanned aerial vehicles (UAVs) were detected flying from the Cambodian side on Sunday night.

The ceasefire took effect at noon local time on Saturday. Both sides agreed to freeze the front lines where they are now, ban reinforcements and allow civilians living in border areas to return as soon as possible.

It had been seen as a breakthrough, which came after days of talks between both countries, with diplomatic encouragement from China and the US.

In a statement on Monday, the Royal Thai Army said Cambodia's actions constitute provocation and a violation of measures aimed at reducing tensions, adding that they were inconsistent with the terms of the ceasefire.

It also said it may need to reconsider the release of 18 Cambodian soldiers held in Thailand since July.

Thailand would be obliged to act in accordance... [if] violations of agreements and national sovereignty continue, it added.

Cambodia has not yet commented.

It comes just hours after China's Foreign Minister praised the hard-won ceasefire, and US President Donald Trump praised the rapid and fair conclusion.

The dispute between Thailand and Cambodia is not new, dating back more than a century. The latest tensions ramped up earlier this year, after a group of Cambodian women sang patriotic songs in a disputed temple, culminating in clashes that left dozens dead and thousands displaced.