Director-general Chalermsak Vanichsombat said so many tourists had gone to mountainous national parks such as Nakhon Ratchasima's Khao Yai and Phu Kradung in Loei that service centres had become very busy, while plants and wildlife were affected by the stream of visitors.

Khao Yai was full beyond capacity last weekend with some tent areas - each of which can take up to 500 people - packed with 1,200-1,500 visitors. This created more garbage, more noise and other troubles, Chalermsak said.

Wildlife in Khao Yai were disturbed and some reportedly began behaving strangely.

For example, tourists wanting to see elephants up close lured them out with a truck loaded with pineapples. This caused many to seek food from passing tourists' pick-up trucks.

And some deer that sorted through garbage for food died after eating and swallowing plastic bags, he said.

The presence of so many people in Khao Yai stressed its ecosystem. Long-tailed macaques stole tourists' belongings and food, while jackals were found going through the piles of garbage, Chalermsak said.

He urged visitors not to feed the animals, saying it would cause changes in their behaviour. And people should be careful not to disturb the park's fragile ecosystem by playing music too loud or visiting merely for somewhere new to have a drinking party.