WASHINGTON (AFP) - US Defense Secretary Robert Gates acknowledged in an interview with Al Jazeera that civilian casualties have become a real problem for the NATO-led mission in Afghanistan. Gates remarks, in an interview to be aired Monday by the Qatar-based Arabic satellite news channel, came amid a raging controversy over an air strike that killed scores of people Friday in northern Afghanistan. I think its a real problem, and General McChrystal thinks its a real problem, too, Gates said, referring to Stanley McChrystal, the commander of US and NATO forces in Afghanistan. According to a transcript of the interview posted on Al Jazeeras website, Gates said the Taliban actively targeted civilians or put them at risk in other ways. But we are trying to figure out new tactics that minimize this. But it is a challenge, he added. Central to the success of the 42 nations that are trying to help the Afghan people and government at this point is that the Afghan people continue to believe that we are their friends, their partners and here to help them. So civilian casualties are a problem for us and we are doing everything conceivable to try and avoid that, he said. At least 54 people were killed Friday when a German commander in Kunduz ordered an air strike on two fuel trucks that had been hijacked by Taliban insurgents. Reports that civilians were among the dead set off a firestorm of criticism, which in turn raised the heat on the government of German Chancellor Angela Merkel, who has insisted the dead were Taliban fighters. The anguished debate in Germany, which only began participating in international military missions a decade ago, comes 20 days before German goes to the polls and amid already meagre public support for the deployment. It also has added fuel to a debate here over the size and scope of the US military commitment in Afghanistan, which has come under scrutiny in the US Congress at a time when commanders, worried about a deteriorating security situation, are expected to ask for more troops. Gates said he had concerns about sending more troops to Afghanistan for fear that Afghans at some point would regard them as occupiers. General McChrystals point, which I think has great validity, is: its really how those forces are used and how they interact with the Afghan people that determines how they are seen by the Afghans. And I think that the approach that he has taken, in terms of partnering with the Afghans, and interacting with the Afghan people, and supporting them, mitigates the concerns that I had, he said. Gates was asked about a recent comment by US envoy Richard Holbrook, who when asked to define success in Afghanistan said, We will know it when we see it.I probably would have answered the question differently, he said. I would have answered it: I believe that success or progress will be when we see the Afghan national security forces, the army and the police, assuming a greater and greater role in security operations protecting Afghanistan and the Afghan people, so that we can recede, first into an advisory role and then leave altogether, he said.