The government is seeking cooperation with neighbouring countries in the Association of Southeast Asian Nations (Asean) to combat the transboundary haze pollution choking the region.

Prime Minister Prayut Chan-o-cha said the issue must be put on Asean's agenda if a solution is to be found.

Prayut said Foreign Affairs Minister Don Pramudwinai had sent a letter to member countries seeking their support.

"I signed the letter myself. We want to seek their cooperation, particularly in dealing with slash-and-burn agriculture. Meanwhile, Thai farmers must also cooperate," the PM said.

Mr Don said yesterday the ministry has been in regular contact with Myanmar and Laos in recent years to reduce transboundary haze.

Mr Don said he met with the ambassadors of the two countries yesterday and impressed upon them the urgent need to work together on this.

The Thailand Environment Institute said the failure to curb the border-crossing haze is partly due to a lack of serious legal enforcement among Asean countries.

"All they can do is seek cooperation. While we are trying to ban the burning of plants, there are also some Thai businessmen investing in monoculture [agriculture] in neighbouring countries," it said.

Chiang Rai saw the worst level of PM2.5 yesterday at 530 microgrammes per cubic metre in Mae Sai district.