Thousands of passengers were stranded across Thailand on Wednesday as train drivers staged a rare wildcat strike to protest against a trade pact with Japan that could threaten their jobs, a senior rail official said.

Drivers refused to work on express trains in the south of Thailand and in the northern city of Chiang Mai, while international services connecting Thailand with Butterworth in northern Malaysia were affected.

Bancha Kongnakorn, acting governor of State Railway of Thailand, said he believed drivers were upset by an imminent free trade pact with Japan which would allow private companies to operate on Thailand's state-owned railways.

"There were 170 drivers -- we estimate 15 percent of the total train drivers in Thailand -- who suddenly took sick leave," Bancha told AFP.

"So far there is no demand set. I have no idea who is leading the strike, but we will clarify with the train drivers that their jobs will be secure -- private freight carriages still need locomotive drivers to pull them."

Bancha said railway executives were negotiating with unions to end the strike and resume services.

The free trade pact between Japan and Thailand is due to take effect on Thursday after years of negotiations.

It tears down most tariffs between the countries, but has been slammed by environmentalists in Thailand, who say it will turn their country into a dumping ground for Japan's toxic waste.