Top rival Bernie Sanders accused Clinton of being too quick to push for regime change, while Republican candidate Carly Fiorina, said the former secretary of state was too willing to cede US policy to other countries.

The debate was the Democrats' first since the deadly December 2 attack by a pair of radicalized Muslims in San Bernardino, California, which along with the November attacks in Paris elevated national security to the top of the campaign agenda.

Obama has relied heavily on US air strikes against Islamic State targets in Syria and Iraq, and has deployed dozens of special operations troops to northern Syria to advise opposition forces. In Iraq, about 3,500 US troops are assisting Iraqi forces.

Like Obama, Clinton backs the use of special forces but says a large deployment of US ground forces in the Middle East would be counterproductive.

"We now finally are where we need to be. We have a strategy and a commitment to go after ISIS," she said, noting a UN Security Council resolution had brought 'the world together to go after a political transition in Syria.'

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