Speaking at a conference of NATO foreign ministers in Berlin, she said, "We need to worry less about how fast we can leave and more about how we can help the Afghan people build on the gains of the past 15 months."
"We have to steel ourselves and our publics for the possibility that the Taliban will resort to the most destructive and sensational attacks we have seen. We have to send a clear message that we remain united," she added.
According to the initial plans by NATO, the handover was to start at the end of 2010, but was hampered by a slow progress in bulking up Afghan forces and by a sudden spurt in insurgent violence, which was at its worst levels since the overthrow of the Taliban in 2001.
With growing weariness in the US about the decade-long war in Afghanistan, Clinton reaffirmed that they would start drawing down its forces from July. President Barack Obama has also endorsed NATO's plan to gradually phase out forces from Afghanistan to complete the transition by 2014.
She also condemned the burning of a Koran by a radical fundamentalist US Christian pastor in March 2011 and said, "We deeply regret that hateful gesture and deplore the inexcusable attacks that followed on the UN's compound in Mazar-e-Sharif and innocent Afghan civilians."
The Koran burning incident triggered massive violence in Afghanistan leaving many dead.