After a meeting with top brass at the Pentagon, Obama said there would be no easy victory against the IS group but said a growing international coalition was resolved to confront the Sunni extremists rampaging in Iraq and Syria.

"Our strikes continue, alongside our partners. It remains a difficult mission," Obama, flanked by the country's most senior military officers, told reporters.

"As I've indicated from the start, this is not something that is going to be solved overnight."

Obama said there was "a broad-based consensus not just in the region but among nations of the world that ISIL (IS) is a threat to world peace, security and order, that their barbaric behavior has to be dealt with."

A Pentagon spokesman earlier offered a sober battlefield assessment, saying US air power on its own could not rescue the town of Kobane from an offensive by the IS jihadists in northern Syria.

US-led aircraft were hitting the IS group at every opportunity but without a competent force on the ground to work with, there were limits to what could be accomplished by bombing from the air, spokesman Rear Admiral John Kirby told reporters.

"Air strikes alone are not going to do this, not going to fix this, not going to save the town of Kobane," Kirby said.

"We know that. And we've been saying that over and over again."

Ultimately, "capable" ground forces -- rebel fighters in Syria and Iraqi government troops -- would have to defeat the IS group, but that would take time, he said.

Kirby said that "we don't have a willing, capable, effective partner on the ground inside Syria right now. It's just a fact."

Other towns could also fall to the IS group until local ground forces could find their footing, he added.

He spoke after American-led forces carried out six air raids near Kobane on Tuesday and Wednesday in an attempt to help Kurdish militia who have fought a desperate battle to hold off the IS militants.

The chairman of the US Joint Chiefs of Staff, General Martin Dempsey, said tracking the militants presented a challenge.

"We have been striking when we can," Dempsey told ABC News in an interview.

IS fighters are "a learning enemy and they know how to maneuver and how to use populations and concealment," the general said.

"They're becoming more savvy with the use of electronic devices," he said.

- Trying to halt IS momentum -

Washington's air campaign, launched in Iraq on August 8 and extended into Syria on September 23, was designed to halt the advance of the IS group to buy time to build up "moderate" rebel forces in Syria and Baghdad government and Kurdish troops in Iraq.

But despite the bombing raids, the IS jihadists have continued to gain ground in both countries, including around the key town of Kobane near the Turkish border.

US and allied bombers, fighter jets and robotic drones hit the IS group on Tuesday and Wednesday with four strikes south of Kobane, destroying an armored personnel carrier, three vehicles and an artillery piece, the military's Central Command said in a statement.

A fifth raid southwest of Kobane destroyed an IS armed vehicle, it said. A sixth strike decimated an artillery cannon on the "southern edge" of the town.

Since September 27, US-led aircraft have conducted 20 strikes near Kobane, according to figures from Central Command.

Coalition warplanes on Wednesday also bombed IS positions elsewhere in Syria, with two strikes northwest of Raqa, hitting a training camp, and a raid in Deir Ezzor, destroying a tank.

Aircraft from the United Arab Emirates, one of five Arab countries involved in the air campaign, took part in the latest strikes along with American planes, said Central Command, which oversees US forces in the Middle East.

Coalition planes also renewed air strikes on IS militants in Iraq on Tuesday and Wednesday, with five bombing raids by fighter jets and unmanned drones, Central Command said.

The operation included three strikes west of Baghdad, where Iraqi government forces are under mounting pressure from IS fighters.