Mr Rasmussen said transition did mean abandoning Afghanistan
Nato's secretary general has said members will do "substantially more" to contribute to the fight against al-Qaeda and the Taliban in Afghanistan.
Anders Fogh Rasmussen said non-US Nato countries would provide at least 5,000 extra troops, and "probably a few thousand on top of that".
His pledge came as President Barack Obama said he would increase US forces in Afghanistan by 30,000 - to 100,000.
Mr Rasmussen said Nato troops would remain there "as long as it takes".
"In 2010, the non-US members of this mission will send at least 5,000 soldiers and probably a few thousand on top of that," Mr Rasmussen said, clarifying that this number was in addition to the more than 38,000 already there.
"What is happening in Afghanistan is a clear and present danger to our citizens," he said. "Instability in Afghanistan means insecurity for all of us."
Earlier on Tuesday, US President Barack Obama announced a surge in the number of US troops and promised to "seize the initiative" in the fight against the Taliban and Al-Qaeda in the country.
He said a pullout would begin in 19 months time, in July 2011, and suggested that the mission in Afghanistan would test the credibility of Nato.
In response, Mr Rasmussen said that the organisation must "now demonstrate that multilateralism produces results".
Mr Rasmussen said the exact contributing nations and precise numbers would be announced after a conference on Afghanistan called by the British prime minister and to be held in London on 28 January.
He said Nato's strategy was "to transfer lead responsibility to running their own country to Afghanistan as soon as possible".
But Mr Rasmussen made clear Nato forces would continue to take a "supporting role".
"We have no intention to have come this far to falter before the finish line. We will stay as long as it takes to finish the job and our mission will end when the Afghans are capable to secure and run the country themselves," he added.
In his weekly address from the Nato headquarters in Brussels, he suggested that there were already some 10-15 districts in the country where Afghan security officials would be ready to take the lead in 2010.