"So what we're looking for is a comprehensive strategy. And there's got to be an exit strategy," Obama said.
"But we can't lose sight of what our central mission is," he said.
"These folks can project violence against US citizens, and that's something we can't tolerate"
Saying that the mission was the same as when the US went into Afghanistan after the September 11, 2001 attacks on the US, he said: "These folks can project violence against US citizens, and that's something we can't tolerate."
Obama's comments come as his administration prepares to roll out its new strategy for Afghanistan amid rising violence blamed on the Taliban movement that the US ousted.
The US president said last month's decision to send 17,000 more US troops to Afghanistan was the most difficult he has had to make since taking office in January.
"You know I think it is the right thing to do. But it's a weighty decision because we actually had to make the decision prior to the completion of [the] strategic review that we were conducting," he said.
Plan to 'bypass Karzai'
US recriminations against the administration of Hamid Karzai, the Afghan president, over its corruption and incompetence, have grown steadily as the military situation has deteriorated.
In an article published by The Guardian, a British newspaper, a new strategy could also be linked to plans made by the US and its European allies "... to plant a high-profile figure in the heart of the Kabul government in a direct challenge to the Afghan presidnent".
While the Obama administration has declared a "realistic" assessment of the Kabul government, they believe the move is needed to transform Afghanistan into a "flourishing democracy".
The Guardian said the idea of a more dependable figure working alongside Karzai is one of the proposals to emerge from the White House review of Afghanistan and Pakistan that was completed last week.
No names have emerged for the new role, but the paper said the US has high regard for Mohammed Hanif Atmar, the country's interior minister.
Other recommendations from the review include increasing the number of Afghan troops from 65,000 to 230,000 as well as expanding the 80,000-strong police force.