Thailand has rejected mediation efforts from third countries to end the ongoing conflict with Cambodia, insisting that Phnom Penh cease attacks and resolve the situation only through bilateral talks, its foreign ministry said on Friday.

Simmering border tensions between Thailand and Cambodia have flared into open hostilities at multiple locations along the frontline, with exchanges of artillery for a second straight day.

At least 16 people, most of them Thai civilians, have died so far in the heaviest fighting between the Southeast Asian neighbours in over a decade.

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.

Cambodia and Thailand accuse each other of starting the conflict early on Thursday at a disputed site, which quickly escalated from small arms fire to heavy shelling along a border where sovereignty has been disputed for more than a century.

“We stand by our position that 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.”

Cambodia’s government did not immediately respond to a request for comment. Its Prime Minister Hun Manet asked the United Nations Security Council on Thursday to convene a meeting on the issue, condemning what he called “unprovoked and premeditated military aggression” by Thailand.

The body has said it will 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 on Wednesday and expelled Cambodia’s envoy, in response to landmine explosions that injured Thai soldiers.

Thai authorities allege the mines had been laid recently by Cambodia, a charge dismissed by Phnom Penh as baseless.

Malaysian Prime Minister Anwar Ibrahim, the chair of 10-nation ASEAN of which both Thailand and Cambodia are members, said on Thursday he had spoken to the leaders of both countries and urged them to find a peaceful resolution.

“If the ASEAN family wants to facilitate a return to constructive bilateral negotiations, that’s welcome as well,” Nikorndej said.