Fighting must first stop between Thailand and Cambodia before there can be negotiations, Thailand's acting premier said on Thursday.

Caretaker Prime Minister Phumtham Wechayachai told a press conference there had been no declaration of war and conflict was not spreading into more provinces.

Earlier, a Thai F-16 fighter jet bombed targets in Cambodia, both sides said, as weeks of tension over a border dispute escalated into clashes that have killed at least 12 people, including 11 civilians.

Of the six F-16 fighter jets that Thailand readied to deploy along the disputed border, one of the aircraft fired into Cambodia and destroyed a military target, the Thai army said.

Both countries accused each other of starting the clash early on Thursday.

"We have used air power against military targets as planned," Thai army deputy spokesperson Richa Suksuwanon told reporters.

Thailand's health minister said 11 civilians, including a child, and one soldier were killed in artillery shelling by Cambodian forces while 24 civilians and seven military personnel were wounded.

Thailand also closed its border with Cambodia.

Cambodia's defence ministry said the jets dropped two bombs on a road, and that it "strongly condemns the reckless and brutal military aggression of the Kingdom of Thailand against the sovereignty and territorial integrity of Cambodia".

There was no immediate word of casualties in Cambodia.

Prime Minister Anwar Ibrahim of Malaysia, the current chair of the Asean bloc in which Thailand and Cambodia are also members, urged calm and said he would speak to leaders of both countries to peacefully resolve their dispute.

China also expressed concern at the fighting and said it was willing to play a role in promoting de-escalation. (Reuters)