Thousands of passengers were stranded across Thailand on Wednesday as train drivers staged a rare wildcat strike that unions said was prompted by mismanagement of the railways.

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.

"There were 170 drivers -- we estimate 15 percent of the total train drivers in Thailand -- who suddenly took sick leave," said Bancha Kongnakorn, acting governor of State Railway of Thailand (SRT).

He said drivers were concerned that their jobs would be threatened by an imminent free trade pact with Japan, which would allow private companies to operate on Thailand's state-owned railways.

"We will clarify with the train drivers that their jobs will be secure -- private freight carriages still need locomotive drivers to pull them," he said.

However, the State Railway Workers Union of Thailand issued a statement saying the train drivers were on strike to protest incompetence by SRT executives and the board, resulting in losses of 23 billion baht (725 million dollars).

"The problems have not been properly addressed. This is the key reason that workers are striking today, to demand government action to solve the problems and prosecute SRT executives," the statement said.

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

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.