US officials "urge all sides to refrain from violence, exercise restraint and respect the rule of law," State Department spokeswoman Marie Harf told reporters.
"We do, I would note, applaud the restraint shown thus far by government authorities in this regard," she said.
Harf said that US officials were working with a "full range" of players "to encourage dialogue and a peaceful democratic transition."
Thousands of protesters have swarmed Bangkok to urge Prime Minister Yingluck Shinawatra to quit and make way for an unelected "people's council" that would oversee reforms to curb the political dominance of her billionaire family.
Thailand is the oldest US ally in Asia, with Washington signing a treaty with the kingdom then known as Siam in 1833.
But recent US diplomacy aimed at resolving Thailand's polarization has had a mixed reception, with a government in 2010 lodging a protest after a US envoy met pro-Thaksin political players.