"It makes me more determined to make sure we're getting our troops home," Obama said in an interview with KDKA, a CBS affiliate in Pittsburgh.
"It's time. It's been a decade, and, frankly, now that we've gotten (Osama) bin Laden, now that we've weakened al Qaeda, we're in a stronger position to transition than we would have been two or three years ago," he added.
But Obama also said he did not think there should be a "rush to the exits" and that the drawdown must be carried out in a responsible way.
Obama will host NATO leaders for a summit in May that will focus on defining plans for gradually trimming Western forces and putting Afghan troops in charge of security.
Most Western forces are due to be gone from Afghanistan by the end of 2014.
Obama was pressed in a separate interview with WFTV, an ABC affiliate in Orlando, on whether there were parallels between the killing of 16 Afghan villagers and the notorious 1968 My Lai massacre of the Vietnam War.
"It's not comparable," Obama said.
"It appeared you had a lone gunman who acted on his own," he said of the Afghanistan incident. "In no way is this representative of the enormous sacrifices that our men and women have made in Afghanistan."
Meanwhile, the US Army staff sergeant accused of the massacre was treated for traumatic brain injury after being in a vehicle that rolled over in Iraq in 2010, a US official says.
The official, speaking on condition of anonymity, said it was premature to state whether there was any link between the 2010 injury and the Afghanistan incident.
