WASHINGTON (AFP) - The Thai military's declaration of martial law must be temporary and not undermine democracy, the United States said late Monday.

The US is concerned about the political crisis in Thailand and urges "all parties to respect democratic principles, including respect for freedom of speech", State Department spokesman Jen Psaki said in a statement.

"We understand the Royal Thai Army announced that this martial law declaration is not a coup. We expect the Army to honour its commitment to make this a temporary action to prevent violence, and to not undermine democratic institutions," Psaki said.

"The United States firmly believes all parties must work together to resolve differences through dialogue and find a way forward. This development underscores the need for elections to determine the will of the Thai people," she added.