WASHINGTON (AFP/Reuters) - Defeating Al-Qaeda requires turning the tide against insurgents in Afghanistan, US Defence Secretary Robert Gates said on Wednesday, arguing for a surge of US forces to take on the Taliban. The first of an additional 30,000 US troops deploying to Afghanistan will begin to arrive in two to three weeks, he said. Rolling back the Taliban is now necessary, even if not sufficient, to the ultimate defeat of Al-Qaeda, Gates told a Senate hearing a day after President Barack Obama unveiled plans to send 30,000 additional troops to Afghanistan. Defending Obamas decision, Gates said it was vital to halt the momentum of the insurgents and that ceding Afghanistan to the militants would hand a powerful propaganda victory to Al-Qaeda. But he also said the Kabul government needed to understand that US forces would not stay indefinitely, and argued that setting a date for the start of an American withdrawal in July 2011 was needed to convey a sense of urgency for Afghan leaders. He said the US is committed to start transferring security responsibility to Afghan forces in July 2011 but may not begin to scale back the US troop surge until later. The 30,000 troop surge could last between 18 and 24 months, giving the Pentagon leeway to assess conditions on the ground before pulling out, he added. Failure in Afghanistan would mean a Taliban takeover of much, if not most, of the country and likely a renewed civil war, Gates said. Taliban-ruled areas could in short order become, once again, a sanctuary for Al-Qaeda as well as a staging area for resurgent militant groups on the offensive in Pakistan. The US Defence Secretary said a victory for the Taliban would strengthen the Al-Qaeda narrative, providing renewed opportunities for recruitment, fund-raising, and more sophisticated operations. Gates was joined at the hearing by Secretary of State Hillary Clinton and the Chairman of the US Joint Chiefs of Staff, Admiral Mike Mullen, who sought to counter arguments against a troop surge. The admiral said the Taliban insurgency has achieved a dominant influence in 11 of Afghanistans 34 provinces. But he told the US Senate Armed Services Committee that the 30,000 additional deployments would allow the US military to gain the initiative in Afghanistan. We believe the insurgency has achieved a dominant influence in 11 of Afghanistans 34 provinces, Mullen said. The admiral acknowledged that more casualties could be expected with the influx of more troops and dismissed assertions that the Taliban would not necessarily support Al-Qaeda if the insurgents took back power. Those who made that argument were ignoring both the recent past and the evidence we see every day of collusion between these factions on both sides of the Afghanistan-Pakistan border, Mullen said. Clarifying the plan in testimony to a congressional committee, Gates said a full scale re-evaluation of where we stand would take place in December 2010. The administration would then assess whether plans to begin transferring security responsibility to the Afghans in July 2011 remained on track. Gates called the July 2011 transition date a clear statement of his (Obamas) strong intent. It is our plan to begin this transition process in July 2011. If circumstances dictate in December (2010), I think as I said the president always has the freedom to adjust his decisions, Gates told the Armed Services Committee. Hillary backed an Afghan-led effort to bring in moderate Taliban who renounced violence.