TOKYO (AFP) - Japan's chief government spokesman said on Tuesday that Tokyo had "grave concerns" about events in Thailand, hours after martial law was declared there.

"We have grave concerns about the situation in Thailand," Chief Cabinet Secretary Yoshihide Suga told reporters in Tokyo.

"We once again strongly urge all parties concerned to act in a self-restrained manner without using violence."

"We strongly hope differences among the parties concerned will be settled peacefully through the democratic process."

Japan is one of Thailand's biggest investors, with several major manufacturers having plants there.

Automaker Nissan told AFP in Tokyo: "Currently we are assessing the situation."

Mr Suga added Japan's government was making utmost efforts to ensure the safety of Japanese nationals in Thailand.