American officials put a warning on the embassy's website quoting Bali's governor as saying: "There is an indication of an attack to Bali tonight."

The alert comes six months after twin suicide blasts killed seven people at luxury hotels in the capital Jakarta.

Bali Governor's I Made Mangku Pastika called on people to be alert, but gave no details about the nature of any attack.

"There is an indication of an attack to Bali tonight, but please don't panic, but put your security system to full alert," he said.

Indonesia's counter-terrorism unit says it received a warning but could not verify its accuracy.

Tourists on the party island have been targeted by Islamic militants, with more than 220 people killed in suicide bombings in recent years.

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