LONDON (Agencies) - The top US military officer says Pakistan is justified in being angry about Nato airstrikes that killed 24 Pakistani soldiers. But he declined to apologise, citing the need for an investigation. In an interview with British television network ITV broadcast late Monday, US Joint Chiefs of Staff Chairman General Martin Dempsey says Islamabad has a reason to be furious that the weapons that killed the Pakistani troops on Saturday were the ordinance of a partner. Speaking on a visit to London, General Dempsey said he would like to enlist the Pakistani governments patience to find out what happened in the incident, in which Nato aircraft struck several Pakistani border posts. The airstrikes happened in an area of poorly-marked and disputed borders between Afghanistans Kunar province and the Pakistani tribal region of Mohmand. Dempsey says US-Pakistani relations are on about as rocky a road as I have seen. But, he said the situation is not irretrievable and military-to-military ties remain solid due to common interests such as fighting terrorism. Dempsey said ties at senior levels between the two nations militaries were still strong at the person-to-person level. He said he had known Pakistans army chief General Ashfaq Kayani since the two studied together at the US Army Command and General Staff College at Fort Leavenworth, Kansas in the late 1980s. Dempsey refrained from repeating some of the accusations about Pakistani intelligence ties to anti-US militants that were cited by his predecessor, Admiral Mike Mullen. Whether they are acting at the behest or at the direction of the ISI - Im not prepared to say that, he said. Pakistan shut Nato supply routes into Afghanistan in retaliation for the killings. Dempsey said the US could cope with the cut-off by channelling supplies through alternative routes. But Id like to believe that we could, over time, with Pakistans approval, restore those lines of communication, he said. Pakistan has also said it had ordered the United States to vacate a drone base in the country. Dempsey, who declined to acknowledge the use of drones at the base, said the move would be a serious act in terms of our relationship. They want us to close the base in Shamsi, the purpose of which I leave to your imagination. There are other options for stationing aircraft and other resources around the region, Dempsey said. Asked whether it was a serious blow, he said: Its a serious blow in the sense that the Pakistani government felt that they needed to deny us the use of a base that weve been using for many years. And so its serious in that regard. Its not debilitating militarily. On the other hand, the spokesman for Isaf on Tuesday said Pakistans decision of asking the US to vacate Shamshi airbase within 15 days should be honoured. The spokesman added that the decision was a matter of Pakistans sovereignty. During a press briefing the spokesman expressed his hope that the government of Pakistan would join the Nato attack investigation and both parties would be united in their quest to build regional security. The US military said Monday an Air Force general will lead an investigation of the incident and compile an initial report by December 23. It says Brig Gen Stephen Clark, a one-star air force general based in Florida, will work with representatives of Nato and the Afghan and Pakistani governments to determine how to prevent similar situations in the future. The team is yet to arrive in Afghanistan but an initial military assessment team went to the border at the weekend after Saturdays catastrophic strike killing 24 Pakistani troops. A Western military official in Kabul, speaking on condition of anonymity, said it was not unusual for US Central Command to carry out such investigations although Nato generally undertakes probes into civilian casualties. The source added that the schedule for the findings being delivered was way quicker than initially expected. Islamabad insists that the air strikes were unprovoked, but Afghan and Western officials have reportedly accused Pakistani forces of firing first. The American news agency, Associated Press, quotes unnamed US officials as saying military investigators believe Taliban militants attacked a US-Afghan patrol in the border region to try to create confusion and draw US and Pakistani forces into firing on each other. With the kind of technology available to the US and Nato, it was expected they would be able to do it (the investigation) much earlier, not more than two weeks, defence analyst Talat Masood told AFP. To reopen the border, there has to be some assurance from the top US leadership, he added. Maybe after such an assurance, the border will be opened or limited supplies allowed (to cross). In an interview with CNN, Prime Minister Yousuf Raza Gilani stopped short of threatening to break the alliance altogether saying: That can continue on mutual respect and mutual interest. US officials, who spoke on condition of anonymity, said the Obama administration was working on a response to a number of demands from Pakistan but said they do not anticipate significant changes in the relationship. White House spokesman Jay Carney said President Barack Obama believed Saturdays incident was a tragedy, and said Washington valued what he called an important cooperative relationship that is also very complicated.