Washington (CNN) President Donald Trump warned the Syrian regime on Wednesday that "the world is watching" as government forces prepare to assault the last rebel stronghold of Idlib.

"They will hopefully be very, very judicious and careful. Because the world is watching. That cannot be a slaughter. If it's a slaughter, the world is going to get very, very angry. And the United States is going to get angry, too," Trump said in the Oval Office when asked about the situation in Syria. "I am watching that very closely."

The President also denied ever ordering or considering ordering the assassination of Syrian dictator Bashar al-Assad when he was pressed Wednesday about the allegation, which is included in a new book by veteran journalist Bob Woodward.

"Not at all. The book is fiction," Trump said. "I heard somewhere where they said the assassination of President Assad by the United States. Never even discussed."

"It was never even contemplated, nor would it ever be contemplated," Trump added.

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