President Barack Obama said Wednesday that it was too soon to say whether a US-led military coalition has the upper hand against the Islamic State jihadist group in Iraq and Syria.

"I think it's too early to say whether we are winning because as I said at the outset of the ISIL campaign, this is going to be a long-term plan," said Obama, speaking to reporters after a crushing Republican victory in Tuesday's midterm elections.

Obama, who approved military action against IS jihadists in Iraq in August before extending air strikes to Syria the following month, warned that a lengthy military campaign lay ahead.

The US president's administration has been accused by critics of bungling its strategy to tackle the IS group, which stunned analysts with a series of sweeping advances across northern Iraq in June.

Obama, who was accused of underestimating IS fighters in January when he appeared to compare them to a second-string high school basketball team in an interview with the New Yorker, said the key to destroying the group's presence in Iraq lay in strengthening the local government and security forces.

The US-led coalition is seeking to "solidify the Iraqi government, to solidify their security forces, to make sure that in addition to our air cover that they have the capacity to run a ground game that pushes ISIL back from territories that they had taken," Obama said.

The coalition was "on the ground providing the training, providing the equipment, providing the supplies that are necessary for Iraqis to fight on behalf of their territory."

Rolling back Islamic State fighters in Syria would prove harder as there was no obvious candidate to act as a proxy force on the ground, said Obama, who has ruled out sending US troops to fight a ground war in Syria.

"There is a specific issue about trying to get a moderate opposition in Syria that can serve as a partner with us on the ground. That's always been the hardest piece of business to get done," said Obama.

"There are a lot of opposition groups in Syria along a spectrum, from radical jihadists who are our enemies to folks who believe in inclusive democracy and everything in between. They fight among each other.

"What we're trying to do is to find a core group that we can work with, that we have confidence in, that we vetted, that can help in regaining territory from ISIL and then ultimately serve as a responsible party to sit at the table in eventual political negotiations that are probably some ways off in the future."