Thailand and Cambodia exchanged heavy artillery fire for a second day on Friday as border fighting intensified and spread, while Cambodia's leader said Thailand had agreed to a Malaysian ceasefire proposal but then backed down.

Both nations accused each other of starting the military clashes and have downgraded their diplomatic relations since Wednesday. Thailand also sealed all land border crossings with Cambodia.

At least 12 have been killed in violent tensions along the disputed border separating Thailand and Cambodia. This follows a flare-up in May in which troops exchanged fire.

Thai health authorities say 14 people killed in Cambodian attacks; Thailand carries out cross-border air attacks.

Thailand and Cambodia exchanged heavy artillery fire on Friday as their worst fighting in more than a decade stretched for a second day, despite calls from the region and beyond for an immediate ceasefire in an escalating border conflict that has killed at least 16 people.

Cambodia and Thailand forces fought each other for the third day over contested territory as the death toll rose to at least 32 as both sought a cease-fire.