President Asif Ali Zardari Saturday welcomed the new US strategy in the war on Islamist militants as a "positive change" and vowed to prevent Pakistan being used for terror activities. "I welcome President (Barack) Obama's call to the Congress to pass the bill for the 1.5 billion dollars aid to Pakistan in a year," he said in a keynote speech to parliament, reiterating comments he made late Friday. "The US presidency's approach now represents a positive change. It is an endorsement of our call for economic and social uplifts as a mean to fighting extremism," Zardari added. Obama on Friday put Pakistan at the centre of the fight against Al-Qaeda in unveiling a new strategy to commit thousands more troops and billions of dollars to the Afghan war and tripling US aid to Pakistan. "Pakistan must demonstrate its commitment to rooting out Al-Qaeda and the violent extremists within its borders," the US president said, throwing down the gauntlet to the nuclear-armed nation.