WASHINGTON - In his first major military decision, US President Barack Obama on Tuesday authorised sending 17,500 additional troops to Afghanistan beginning this spring, a move that appears to mark the formal American troop shift from Iraq to Afghanistan. "There is no more solemn duty as president than the decision to deploy our armed forces into harm's way. I do it today mindful that the situation in Afghanistan and Pakistan demands urgent attention and swift action," Obama said in a statement. "The Taliban is resurgent in Afghanistan, and al Qaeda supports the insurgency and threatens America from its safe haven along the Pakistani border." Obama did not specify the "swift action" he intends to take in dealing with the situation in Afghanistan and Pakistan. Defence Secretary Robert Gates ordered the deployment of 8,000 Marines from Camp Lejeune, NC, and 4,000 Army soldiers from Fort Lewis, Washington, defence department officials said. Approximately another 5,000 support troops are expected to receive deployment orders "at a later date," a Defence Department statement said. The first Marines could arrive in Afghanistan by May. The Army brigade is expected to arrive by mid-summer. There are 38,000 US troops in Afghanistan and 146,000 in Iraq. The magnitude of the deployments will force the military to pull troops from Iraq or give soldiers longer deployments or shorter breaks. In his statement, Obama said that he understood the deployments will place an "extraordinary strain" on the troops, but it will be possible because "we are going to responsibly draw down our forces in Iraq." The decision comes amid a broad review of US policy in Afghanistan that isn't due for completion until just before a NATO summit in early April. Obama said the troop decision "does not pre-determine the outcome of that strategic review." He had to make a decision before then, an unnamed senior administration official was quoted as saying because of the looming onset of the fighting season in Afghanistan with warmer weather, and preparations for national elections there later this year. If the president had not acted now, "the options start to slip away," the official said. Senator John McCain, a former prisoner of war and Obama's Republican opponent in the presidential election last year, said that sending more troops was long overdue but that "I believe the president must spell out for the American people what he believes victory in Afghanistan will look like and articulate a coherent strategy for achieving it." Senate Majority Leader Harry Reid, a Democrat, anticipated the questions about the strategy in his statement. "I support President Obama's approval of a request from the ground commanders for more troops," Reid said. "I also strongly support the comprehensive strategic review of our policy that is currently under way." John McHugh, the ranking Republican on House Armed Services, echoes that theme. "While the deployment of additional US personnel is welcome, our commanders on the ground and the secretary of Defence have consistently indicated that additional troops will be required in the future," McHugh said in a statement. "However, President Obama should not assume that more troops are the complete answer to the challenges in Afghanistan. We need to involve all elements of national power in this struggle and develop a balanced, comprehensive strategy for the region and the issues involved." An advocacy group for military families articulated another concern afoot in Congress, particularly among Republicans: "We also cannot diminish the tremendous progress the surge has made in Iraq over the past two years by abandoning Iraq to fight in Afghanistan," said Military Families United. House Majority Leader Steny Hoyer, a Democrat, took another tack on that issue. "Democrats have long said that the centre of the war on terror is Afghanistan, and this renewed commitment to our fight there demonstrates the president's appreciation for this challenge," Hoyer said in a statement. "The conflict in Iraq has taken our eye off of a resurgent global network of terrorists, and this action responds to requests from commanders on the ground to increase their ability to effectively combat those who seek to harm us." McCain and other lawmakers also suggested that US allies need to respond to the president's statement by adding their own troops in Afghanistan and by loosening restrictions on how they can be used. "I hope our NATO allies will take our example to heart and provide more assistance as well," said Ike Skelton, a Democrat who is chairman of the House Armed Services Committee.