WASHINGTON (Agencies) - The US is "running out of time" to win the war in Afghanistan, and sending in more troops will not guarantee victory, the Chairman of the Joint Chiefs of Staff warned Congress, saying that he had ordered the military to draw up a new strategy that encompasses insurgent safe havens in Pakistan. In their statement to the House Armed Services Committee on Wednesday, both Adm Mike Mullen and US Defence Secretary Robert Gates described a new emphasis on Afghanistan, included a greater push to improve the Afghan security forces, and increased pressure on Pakistan to work with Kabul to quell insurgents crossing the border. Army Gen David McKiernan, the Nato force commander who Bush nominated Wednesday for reappointment as commander of the International Security Assistance Force Afghanistan, has asked for an additional three brigades, or about 10,000 troops. But Gates said, "Additional forces alone will not solve the problem," and emphasized the need for greater economic and political development and better organised international support. He said insecurity and violence will persist in Afghanistan until the insurgency is deprived of safe havens in Pakistan, but suggested that the United States wanted to avoid confrontation with Islamabad. "During this time of political turmoil in Pakistan, it is especially critical that we maintain a strong and positive relationship with the government, since any deterioration would be a setback for both Pakistan and Afghanistan," he said. Mullen told the panel that while he is not convinced the coalition is winning in Afghanistan, "I am convinced we can." The Admiral said investment and alternative crops will also be key to winning in Afghanistan. He said the US military, faced with growing extremist violence in Afghanistan, will modify its strategy for region to include militant safe havens in Pakistan. "I'm not convinced we are winning it in Afghanistan. I am convinced we can," Navy Adm Mike Mullen, Chairman of the Joint Chiefs of Staff, said at the congressional hearing. He said he was "looking at a new, more comprehensive strategy for the region" that would cover both sides of the Afghanistan-Pakistan border. "In my view, these two nations are inextricably linked in a common insurgency that crosses the border between them," he said. Mullen said he is convinced the war can be won, but the US urgently needs to improve its nation-building initiatives and its cross-border strategy with Pakistan. "We can't kill our way to victory, and no armed force anywhere - no matter how good - can deliver these keys alone. It requires teamwork and cooperation," Mullen said, adding that they (militants) have grown "bolder." Cross-border attacks into Afghanistan by militants in Pakistan's tribal region are a problem, and the US has deployed Predator drones to attack targets in Pakistan. Mullen stressed that Afghanistan can't be referenced without "speaking of Pakistan," where, he said, the militant groups collaborate and communicate better, launch more sophisticated attacks, employ foreign fighters and use civilians as human shields. "In my view these two nations are inextricably linked in a common insurgency that crosses the border between them," he said, adding he had commissioned a new, more comprehensive military strategy for the region that covered both sides of that border. "I have pressed hard on my counterparts in Pakistan to do more against extremists, and to let us do more to help them," he said. The conflict is exacerbated, he said, by the "poor and struggling Afghan economy," as well as the drug trade and "significant political uncertainty in Pakistan." These factors present a "complex, difficult struggle." At the same time, Mullen warned "that no amount of troops in no amount of time can ever achieve all the objectives we seek." Until Afghan security forces gain the backing of local leaders to improve security, "we will only be as much as a crutch - and a temporary one at that," the Admiral said. "We can hunt down and kill extremists as they cross over the border from Pakistan, as I watched us do during a daylong trip to the Korengal valley in July. But until we work more closely with the Pakistani government to eliminate the safe havens from which they operate, the enemy will only keep coming," he said. The Admiral said US and NATO forces in Afghanistan blamed militant safe havens in Pakistan for launching bolder, more sophisticated attacks on US and Nato forces in eastern Afghanistan. Robert Gates said improved security in Iraq will give the US military the flexibility to do more in Afghanistan in the coming months. But even as Gates hinted as possible further troop cuts in Iraq next year, he said that a go-slow approach is justified by several worrisome circumstances, including slow progress on the political front. He said the United States is now in the "end game" in Iraq but must move cautiously in drawing down its forces there despite a growing insurgency in Afghanistan. "I believe we have now entered that end game and our decisions today and in the months ahead will be critical to regional stability and our national security interests for the years to come," he told lawmakers. Mullen called the decision to send 4,500 troops to Afghanistan "a good and important start" even though it fell short of commanders requests for three more brigades or about 10,000 troops. "Frankly, I judge the risk of not sending them too great a risk to ignore," he said at the same hearing before the House Armed Services Committee. Gates acknowledged the rising insurgent challenge in Afghanistan, but he warned that risks of reversals remain in Iraq despite progress on the security front. In Afghanistan, Gates said, "persistent and increasing violence resulting from an organised insurgency is, of course, our greatest concern."