Hillary Clinton has described as “dangerous” her opponent Donald Trump’s assertion that the Iraqi-led assault on Mosul is a “total disaster”, after the Republican presidential nominee tweeted that the operation against the Isis stronghold made the US look “dumb”.

The offensive to reclaim Iraq’s second-largest city from Islamic State fighters began last week after many months of preparation, under the leadership of the Iraqi military. The operation, in which one US service member has already been killed, was expected to take several weeks.

In a tweet sent on Sunday night, however, Mr Trump repeated his previous assertion that Isis was given too much “notice” of the impending attack. The property developer did not offer any specific evidence to back up his analysis.

Speaking at a rally in Manchester, New Hampshire on Monday, Ms Clinton said Mr Trump’s comments demonstrated the dangers of electing an ill-informed and unqualified commander in chief. “He’s basically declaring defeat before the battle has even started,” said the former Secretary of State. “It’s not only wrong, it’s dangerous, and it needs to be repudiated.”

During the third and final presidential debate last week, Mr Trump insisted the offensive on Mosul ought to have been a “sneak attack”, complaining that the US-backed operation lacked “the element of surprise” and suggesting the celebrated American generals Douglas MacArthur and George Patton would be “spinning in their graves”.

In fact, say many military commanders, it is neither unusual nor “dumb” to warn of a major ground offensive ahead of time: it can strike fear into an enemy, and allow civilians to escape the battlefield beforehand, retired US Army colonel Jeff McLausland told the New York Times. “What this shows is Trump doesn’t know a damn thing about military strategy,” he said.

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The Mosul offensive has been planned for at least a year, and involves some 30,000 troops including Iraqi soldiers and police, Kurdish Peshmerga, Sunni and Shiite fighters, with the US providing special forces and air power. The coalition recaptured the Isis-held Iraqi cities of Tikrit, Ramadi and Fallujah before moving on Mosul, a city of one million people that fell to the Islamic State in June 2014.

Polls suggest the public already trusts Ms Clinton more than Mr Trump on foreign policy and tackling the threat of Isis, yet the Republican also claimed at the final debate last Wednesday that the Mosul operation was timed to improve his rival’s presidential chances. “She wanted to look good for the election, so they’re going in,” he said.