Thailand's biggest foreign investor, Japan, said Friday it would offer support to its firms in the kingdom as concerns grow that the death of its revered monarch could derail the economy.

Japan has more than 4,000 companies operating in Thailand, dubbed the "Detroit of Southeast Asia" due to the huge number of auto manufacturing plants.

"The government will offer appropriate support to Japanese companies if necessary so that the impact on them would be kept to a minimum," top Japanese government spokesman Yoshihide Suga told a regular press briefing.

He did not elaborate on what sort of help -- financial or otherwise -- Tokyo might give.

Concerns over the future of the country have been lingering with the ailing health of the world's longest-reigning monarch, who has played a role of mediator at times of political turmoil during his seven-decade reign.

King Bhumibol Adulyadej passed away Thursday at the age of 88 after years of ill health.

"Significant near-term disruption is likely," John Marrett, research analyst at the Economist Intelligence Unit, said in a commentary.

"(We are) expecting a slowdown in Thailand's economy over the next 12 months."

Thai stocks rebounded sharply on Friday after plunging this week as analysts warned over the possible fallout -- political and economic -- from the king's death.

Japanese auto giants Toyota and Honda said it would be business as usual for their factories in Thailand.

But Japanese media have quoted unnamed insiders at firms operating in Thailand who said they worried they might have to shut plants because Thai employees could be reluctant to work as the nation mourns the king's death.

Releasing new products during the one-year mourning period could be seen as being in bad taste, they added.

"Our factories in Thailand will keep operating as usual this week," Toyota spokesman Itsuki Kurosu said of the auto giant's three plants.

Rival Honda also said it would keep its four Thai plants running, including three auto factories and a motorcycle facility.

But "if the Thai royal family or the government issues instructions (to halt operations) then we'll comply with them," added Honda spokesman Tamon Kusakabe.

While much of Japan's investment in Thailand -- about $4.2 billion last year alone -- is based in manufacturing, more and more service-sector firms have opened up in the country in recent years.