Thailand favours bilateral negotiation rather than third-party mediation to resolve its military conflict with Cambodia, two Thai officials have said, as fighting along their disputed border continued unabated, with Thailand declaring martial law in eight of its districts bordering Cambodia.

Simmering border tensions between Thailand and Cambodia have flared into open hostilities at multiple locations along the frontline in the heaviest fighting between the Southeast Asian neighbours in over a decade.

There were exchanges of artillery for a second straight day on Friday.

Apichart Sapprasert, commander of the military's Border Defence Command in the provinces of Chanthaburi and Trat, said in a statement, "Martial law is now in effect" in seven districts of Chanthaburi and one district of Trat.

The United States, China, and Malaysia, which is the current chair of the ASEAN regional bloc, have offered to facilitate dialogue, but Bangkok is seeking a bilateral solution to the conflict, Thai foreign ministry spokesperson Nikorndej Balankura told Reuters.

"I don't think we need any mediation from a third country yet," Nikorndej said in an interview.

Malaysian Prime Minister Anwar Ibrahim, the chair of ASEAN, of which both Thailand and Cambodia are members, said on Thursday he had spoken to both countries' leaders and urged them to find a peaceful resolution.

"We appreciate (the offer), and we don't want to rule out having a third country to help, but right now we believe that bilateral mechanisms have not been exhausted," Thai vice minister for foreign affairs Russ Jalichandra told reporters.

‘Doors are open’

Cambodian Prime Minister Hun Manet said in a social media post late on Friday that both Thailand and Cambodia had initially agreed to a ceasefire proposal from Anwar a day earlier, but Thailand later reversed its decision.

The Thai government did not immediately respond to a request for comment on Hun Manet's remarks.

The two countries accuse each other of starting the conflict early on Thursday along a border where sovereignty has been disputed for more than a century.

"We stand by our position that a bilateral mechanism is the best way out; this is a confrontation between the two countries," Nikorndej said, adding that the Cambodian side must stop violence along the border first.

"Our doors are still open."

On Thursday, Hun Manet asked the United Nations Security Council to address the fighting, condemning what he called "unprovoked and premeditated military aggression" by Thailand.

The body said it would hold a closed-door meeting to discuss the issue on Friday.

The fighting broke out a day after Thailand recalled its ambassador to Phnom Penh and expelled Cambodia's envoy in response to landmine explosions that injured Thai soldiers.

Bangkok has alleged that those were recently laid, which Phnom Penh rejected.