Baghdad- The US has conducted its second air-drop of food and water to thousands of Iraqis hiding in mountains from jihadist fighters, the Pentagon says today. It came hours after the US launched fresh air strikes against militants from the Islamic State (IS). The group had recently made fresh gains in northern Iraq and is threatening the Kurdish city of Irbil. The US is also piling pressure on Iraqi leaders to form a unity government capable of dealing with the jihadists. Sunni Muslim group IS, formerly known as ISIS, has taken control of swathes of Iraq and Syria and has also seized Iraq's largest dam. President Barack Obama hasn't set a timetable for the current intervention and White House spokesman Josh Earnest said simply that the length of the campaign would be determined by events on the ground. He said that, ultimately, a solution to the current violence rests with the formation of an Iraqi government that was representative of all the Iraqi people. Yet the course of this engagement was difficult to predict. White House officials concede they were concerned about the Islamic State fighters' proficiency - they were said to be well armed and well trained - hence it remains to be seen whether air strikes would be effective. All of which raises the prospect of "mission creep" if the current campaign doesn't halt their advance. in a statement, the Pentagon said the latest air-drop involved one C-17 and two C-130 cargo planes that together dispersed a total of 72 bundles of supplies. The cargo aircraft were escorted by two F/A-18s from the aircraft carrier USS George HW Bush, it added. The aid was dropped into the mountains around the town of Sinjar, where up to 50,000 members of the Yazidi religious sect fled an IS advance a week ago. IS fighters also seized Qaraqosh, Iraq's biggest Christian town this week, causing thousands more to flee. The first US air strike on Friday saw two 500lb (227kg) bombs dropped on IS artillery being used against forces defending Irbil. Late on Friday, the Pentagon confirmed a second wave of attacks. It said drones and fighter jets attacked a mortar position and a seven-vehicle convoy carrying fighters also threatening Irbil. IS remained defiant, one fighter told media that the strikes would have "no impact on us". "The planes attack positions they think are strategic but this is not how we operate. We are trained for guerrilla street war," he said. The air strikes are the first time US forces have been directly involved in a military operation in Iraq since American troops withdrew in late 2011. Marie Harf, a spokeswoman for the US state department, said the immediate goal of the strikes was to "prevent the advance" of IS towards Irbil, where US personnel are based.