BAGRAM  - A rocket fired on a US airbase in Afghanistan early on Tuesday damaged the aircraft of America's top military officer and injured two maintenance crew, the officers said.Two insurgent rockets struck the vast Bagram air field overnight, with one causing damage to  C-17 used by General Martin Dempsey, Chairman of the US Joint Chiefs of Staff, who later left the base using another plane.Shrapnel from the rocket struck the door of the plane while it was parked on the runway, with two American maintenance crew suffering minor injuries in the attack, Dempsey's spokesman Colonel Dave Lapan said.The rockets posed no threat to the safety of Dempsey or his staff, who were asleep in their quarters at the time of the incident in the early hours of the morning, officers said. A spokesman for NATO's International Security Assistance Force (ISAF) described the rocket hitting the General's plane as simply a lucky shot. "It was a C-17 that looked just like every other C-17 on the tarmac," he told AFP."Indirect fire is not unusual at Bagram and there is absolutely no indication that this was an attack specifically targeting that plane," the spokesman said. Dempsey had been visiting Kabul to meet commanders of the NATO-led force and Afghan top brass amid a surge in assaults by Afghan security personnel on their international colleagues.President Barack Obama, who spoke to Dempsey before the attack by phone while he was in Kabul, said Monday the United States was watching the rise in insider attacks with "deep concern", telling a White House news conference: "Obviously, we have to do more."NATO and American officers have suggested that the Afghan government has failed to come to grips with the problem but Dempsey said he came away "reassured" after discussions with his Afghan counterpart, General Shir Muhammad Karimi. "I am reassured that the Afghan leaders, military and civilian, understand how important this moment is," Dempsey said.Taliban insurgents have taken credit for the so-called green-on-blue assaults while NATO officers say an internal review showed only about 10 per cent of them were the result of infiltration.NATO has blamed the incidents on a mixture of cultural differences, personal vendettas and propaganda by Islamist militants.The attacks are unprecedented in US military history and they have spawned so much mistrust that foreign troops have been ordered to be armed at all times, even within bases, officers said.Afghan authorities have adopted more rigorous vetting of recruits and NATO has bolstered counter-intelligence but the measures have failed to stem the problem.NATO has about 130,000 soldiers fighting an insurgency by Taliban Islamists, but they are due to pull out in 2014 and now work increasingly with the Afghans they are training to take over.Dempsey, on the first leg of a trip to Afghanistan and Iraq, said the insider violence would not alter the timetable for withdrawal or the coalition's emphasis on cooperating with Afghan recruits.But the growing number of attacks is likely to add to pressure in NATO nations for an early exit from the increasingly unpopular conflict, now nearly 11 years old and America's longest war.