The Pentagon warned Wednesday US air power on its own could not prevent Islamic State jihadists from capturing the Syrian border town of Kobane, even as US warplanes kept up bombing raids in the area.

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.

The Pentagon's sober assessment came 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 group's push.

IS extremists have been closing in on Kobane for days but Kurdish forces -- backed by US-led air power -- reportedly managed to roll back IS militants out of several neighborhoods amid heavy fighting overnight.

The chairman of the US Joint Chiefs of Staff, General Martin Dempsey, said coalition aircraft were bombing the IS group whenever possible but 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.

He indicated the IS extremists were more difficult to track as they were staying off of mobile phones or other devices that could be monitored.

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

"They don't fly flags and move around in large convoys the way they did. . . They don't establish headquarters that are visible or identifiable."

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 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 pressure from IS fighters, it said.

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