Asked by reporters about a possible US attack on Iran, Trump said "there are many options. There's the ultimate option and there are options a lot less than that."

He explained that by "ultimate option" he meant "war."

Trump said that the specifics of sanctions he announced earlier would be made public "over the next 48 hours."

US ally Saudi Arabia says Iran was behind a missile or drone attack setting ablaze major oil facilities last weekend.

Trump has so far appeared to resist calls from some in his Republican party for more a more aggressive response.

"There's plenty of time to do some dastardly things," he said. "If we have to do something we'll do it without hesitation."

Trump also referenced his frequent message that he wants to keep the United States out of conflicts in the Middle East. "How did that work out?" he asked, referring to Iraq and other conflicts.

Although US officials have indicated they clearly believe the Saudi blame placed on Iran, despite Tehran's denial, Trump has yet to state this fully in public.

"We're really at a point now where we know very much what happened," he said, without giving more detail.

Trump had reportedly been hoping for a dramatic encounter with Iranian President Hassan Rouhani at next week's UN General Assembly in New York.

The crisis over the Saudi oil facilities attack appears to have made that meeting highly unlikely. On Wednesday, Iranian state media reported that Rouhani and his delegation were having trouble obtaining US visas.

US Secretary of State Mike Pompeo declined to comment specifically on the case, but suggested that the Iranians should be denied because of what he said was the country's "terrorism."

Trump said "if it was up to me I'd let them come."