WASHINGTON - The joint NATO-Afghan military's anti-Taliban offensive in southern Afghanistan is progressing but more slowly than expected, top U.S. military officials say. Chairman of the U.S. Joint Chiefs of Staff, Navy Adm. Mike Mullen, spoke at a Pentagon joint briefing in Washington about U.S. anti-Taliban offensive launched Feb. 13 in Helmand province. Mullen, who was joined by U.S. Defence Secretary Robert Gates, said the Taliban have put up stronger resistance in some cases than had been anticipated. However, he said the Afghan and coalition forces are making gains. "As you have all been seeing, we are making steady, if perhaps a bit slower than anticipated, progress," Mullen said. The admiral said the real challenge is clearing booby traps and crude bombs planted by Taliban fighters, the report said. Mullen expressed regret and condolence over what NATO said were the accidental deaths of civilians last Sunday while a NATO airstrike targeted suspected insurgents. Saying war is messy and ugly, he said, "We must steel ourselves, no matter how successful we are on any given day, for harder days yet to come." Gates said Gen. Stanley McChrystal, commander of coalition and U.S. forces in Afghanistan, is doing "everything humanly possible" to avoid civilian casualties. "But it is also a fact that the Taliban mingle with civilians," Gates said. He said McChrystal is balancing concerns for Afghan civilians and military forces, adding he would not "try and second-guess" McChrystal "from 9,000 miles away." "He is the commander. I have confidence in his judgment," Gates said. A VOA report said at least 16 civilians, about 120 insurgents and 13 NATO troops have died in the Helmand campaign.