TEL AVIV (Reuters) - A major Nato-led offensive against a Taliban stronghold in Afghanistan began well but its completion is difficult to predict, the United States top military officer said on Sunday. Its off to a good start, Admiral Mike Mullen, Chairman of the US Joint Chiefs of Staff, told reporters during a visit to Israel when asked about progress in the advance on the town of Marjah, now in its second day. Its actually very difficult to predict (the end), he said. We have from a planning standpoint talked about a few weeks, but I dont know that. The 15,000-troop offensive, one of Natos biggest against the Taliban since the Afghan war began in 2001, is designed to extend Kabuls influence over the last big rebel bastion in Helmand province. This is focused on the people, Mullen said. This is not focused on the Taliban, and it is a strategy not just to clear the area but to hold it and then build right behind it so that there is a civilian component here and there is a local governance...that is going to be installed as well.