Tampa, Florida (CNN) Donald Trump isn't letting up on his criticism of the offensive to retake the Iraqi city of Mosul from ISIS. In fact, he doubled down on Saturday, questioning the reasons for the US-backed mission and criticizing US leaders involved in its planning.

The Republican presidential nominee knocked US officials as a "group of losers" for not launching a "surprise" attack and said he was convinced the offensive -- which is led by the Iraqi military -- was launched "for political reasons" to benefit his Democratic opponent Hillary Clinton. He suggested she would get "credit" for its success.

Trump's repeated criticisms of the ongoing military operation in Mosul come despite forces on the ground making key gains.

"Whatever happened to the element of surprise, right?...What a group of losers we have. And now it's a very tough battle, they're dug in. It's a very -- much tougher than they thought," Trump said at a campaign event in Tampa, Florida. "We need different thinking in this country, folks. They should have kept their mouths shut."

Those remarks came after he questioned the core rationale behind the military offensive, which is central to defeating the terrorist group in Iraq.

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