WASHINGTON (REUTERS) - The United States called on Thailand's army to respect "democratic principles" and abide by its commitment not to stage a coup after it declared martial law nationwide on Tuesday.

"We're watching the situation very closely. We expect that the Thai Army will be true to its word when it says that this is not a coup and this is just a temporary injunction," said Pentagon spokesman Rear Admiral John Kirby.

"It's important for everybody to respect the democratic principles there."

Army chief General Prayuth Chan-ocha said the military had stepped in to restore order and build investor confidence after six months of street protests. He warned that troops would take action against anyone who used weapons and harmed civilians.

Military officials insisted the surprise intervention was not a military coup and said they were not interfering with the caretaker government.

But ministers were not informed of the army's plan before an announcement on television at 3am local time and General Prayuth said martial law would be maintained until peace and order had been restored.

The United States is working to strengthen ties with Asian nations including Thailand, and the Thai army's move came the same day that some 700 US Marines and sailors started a military training exercise there, both on the ground and at sea, due to run until next week.

"It's going on as scheduled ... There's been no changes to the exercise," Admiral Kirby said.

US Defence Secretary Chuck Hagel has not spoken to Thai officials since the announcement, Admiral Kirby said.

"They've said it's not a coup, they've said it's temporary. It's our expectation that they'll ... live up to those words," he said.