Hillary Clinton has slammed her rival Donald Trump for saying that the week-old effort to retake the Iraqi city of Mosul from the control of Islamic State is going badly.

"He's basically declaring defeat before the battle has even started," Clinton said at a campaign event in New Hampshire. "He's proving to the world what it means to have an unqualified commander in chief."

In a tweet on Sunday, Trump, the Republican nominee for the November 8 election, said the "attack on Mosul is turning out to be a total disaster. We gave them months of notice. US is looking so dumb."

Iraqi and Kurdish forces, backed by the United States, have mounted a huge assault on the area surrounding the city, the last stronghold of Islamic State forces in Iraq. They have retaken about 80 Islamic State-held villages and towns since the offensive was launched on October 16, but have yet to move on the city itself.

Trump reiterated his position during a rally on Monday in St Augustine, Florida, where he also urged supporters to vote early and declared his campaign was winning the election.

"So now we're bogged down in Mosul. The enemy is much tougher than they thought. They've had a lot of time to get ready," Trump said. "It's a horrible, horrible situation that's going on. Why did we have to tell them we're going in?"

Trump suggested last week during the final 2016 presidential debate that the US- backed attack on Mosul was orchestrated to help Clinton in her White House bid.

With just over two weeks to go until the election, Clinton, President Barack Obama's first-term secretary of state, leads the New York businessman in national opinion polls. Both candidates have been focusing on a small set of political swing states that could decide the contest.

Seeking to cement a wide advantage she holds with women voters, Clinton enlisted the help of firebrand US Senator Elizabeth Warren of Massachusetts, who blasted Trump over allegations he tried to grope or kiss several women without their consent over a 20-year span.

"He thinks because he has a mouthful of Tic Tacs that he can force himself on any women within groping distance," Warren told a raucous crowd of 4,000 at St Anselm College in Manchester. "Well, I've got news for you, Donald Trump. Women have had it with guys like you."

Trump spent the day campaigning in Florida, a critical swing state. At an event in Tampa, he criticised Clinton's position on Syria. "If you look at her plans for Syria, these are the plans of a child. These are the plans of someone who doesn't know what they're doing," he said.

At an earlier event for farmers in Boynton Beach, Trump disputed multiple national and state polls that show him lagging Clinton and accused the media of distorting poll results to discourage his supporters from voting.

"I believe we're actually winning," Trump said.