WASHINGTON: The United States is concerned by the expansion of the Taliban in Pakistan, a senior State Department official told AFP Wednesday after the Islamic militants moved closer to Islamabad. 'This latest move is of concern to us,' the official told AFP on the condition of anonymity. 'But we continue to work with the government of Pakistan to counter Taliban and other violent extremists operating in both Pakistan and Afghanistan.' Pakistani officials said Wednesday that Taliban militants in Pakistan's Swat valley have moved into another district in a bid to broaden their control despite a deal designed to end extremist violence. Hundreds of armed Taliban from the scenic northwestern valley have entered Buner district, only 110 kilometres (68 miles) from the capital Islamabad. Indications of a spread of Taliban activism will likely fuel criticism of the accord agreed by Pakistani President Asif Ali Zardari earlier this month for Swat, which the United States said amounted to capitulation. The top US military commander Admiral Michael Mullen told NBC news from Kabul he was concerned about the threat from Al-Qaeda, which the Taliban hosted while it was in power in Afghanistan during the September 11, 2001 attacks. 'The threat is very real. I see intelligence routinely that indicates that Al-Qaeda is plotting against the United States, against western interests,' he said after he arrived in Afghanistan Wednesday on a three-day visit. He was also concerned about the prospect of both Afghanistan and Pakistan descending into chaos, when queried by the broadcaster. 'Pakistan -- it's a country that has nuclear weapons. My long-term worry is that descent ... should it continue, gives us the worst possible outcome there,' he said. Asked if the US has a plan to safeguard Pakistan's nuclear weapons, he replied: 'I wouldn't go into any kind of detail planning. I'm comfortable right now that the measures that the Pakistanis have taken in recent years provide adequate security.' He said it was still too early to say how long it will take US-led military and other efforts to defeat the Islamists in Afghanistan and Pakistan. 'These next two years, I think, will tell that tale. There will be a significant engagement for a period of time. We're not going to turn it around and succeed in the next 24 months,' he said.