US and coalition warplanes kept up bombing raids on Islamic State jihadists near the Syrian border town of Kobane on Wednesday, as America's top military officer acknowledged difficulties in tracking the militants from the air.

The US military said American-led forces carried out six air raids near the contested town in a 24 hour period in an attempt to help Kurdish militia defending Kobane from an offensive by the IS group.

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 the militants were often concealing their movements.

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

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 some areas, 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.

One raid east of Fallujah destroyed an IS checkpoint and armed vehicle, a strike in western Ramadi destroyed three IS buildings and two anti-aircraft artillery pieces and a bombing run northwest of Ramadi destroyed an IS checkpoint, it said.

Coalition forces, which included British and Dutch warplanes, also bombed IS targets near Mount Sinjar in two strikes.

The coalition has flown more than 4,800 sorties, including refueling and surveillance flights, since the air campaign began on August 8. The United States has conducted an overwhelming majority of the air strikes and other flights, using planes at bases in the region as well as fighter jets on an aircraft carrier in the Gulf.