"I'm going to take a very deliberate process in making those decisions," Obama said at the White House.
"And so I just want to be absolutely clear, because there's been a lot of discussion in the press about this: There is no immediate decision pending on resources... My determination is to get this right."
General Stanley McChrystal, the commander of US and Nato forces in Afghanistan, last week delivered a report to the Obama administration which contained a grim assessment of the war.
McChrystal is widely expected to call for thousands more US troops to be sent to Afghanistan to counter fighters loyal to the Taliban and al-Qaeda, who have been resurgent in the past few months.
Obama has already ordered 21,000 more troops to be sent to Afghanistan, taking the US military presence in the country to 68,000 by the end of 2009.
Violence against US troops is now at its highest level since the war began in October 2001, with 51 soldiers killed in August alone.
Obama's popularity ratings have dropped amid mounting concern among the US public over whether the war in Afghanistan is winnable.
Meanwhile, senior Democrats have expressed concern about the US strategy in Afghanistan, with some saying that too large a US presence in the country could alienate the Afghan population.
Obama said that he will continue to seek a broad assessment of military, diplomatic, civilian and developmental efforts in Afghanistan before he makes any decisions on strategy.
"One of the things that I'm absolutely clear about is you have to get the strategy right and then make determinations about resources," Obama said.
"You don't make determinations about resources - certainly you don't make determinations about sending your men and women into battle - without having absolute clarity about what the strategy is going to be."
Harper, who has 2,500 Canadian troops in Afghanistan, said that while the Taliban has failed to present Afghans with a viable alternative government, "we are concerned about the strength of the insurgency" and whether Afghan forces can provide enough security without foreign assistance.
Canada plans to withdraw its troops in 2011.