transfer security responsibilty to the Afghan government, there could be no question of the alliance rushing for the exit door.
Speaking to reporters, the new secretary-general said that troops serving in what is NATO's biggest ever mission would help prevent Afghanistan from "becoming again a grand central station of international terrorism."
Around 90,000 foreign troops are currently stationed in Afghanistan to counter an insurgency by the Taliban against the government of President Hamid Karzai.
The Islamist militia has stepped up its attacks in the countdown to national elections on August 20, their latest bombing killing 12 people on Monday in the western city of Herat.
Seventy-five foreign soldiers were killed last month according to an independent website, making July the deadliest month for troops since the US-led invasion.
Rasmussen said that the long-term goal was to "move forward concretely and visibly with transferring lead security responsibilty in Afghanistan to the Afghans."
"I believe during my term Afghans must take over lead responsibility for security in most of their country," he added.
But any suggestion that such a strategy amounted to cut and run was pure propaganda, he added.
"Let me be clear. NATO must and will be there in support. Let no Taliban propaganda try to sell my message as a run for exit. It is not," he said. "We will support Afghan people as long as its takes."
In a weekend newspaper interview, the 56-year-old said that he would support dialogue with moderates within the Taliban. The Islamist militia had ruled Afghanistan until late 2001 but it was toppled by US-led forces after it refused to hand over Al-Qaeda leader Osama bin Laden in the wake of the September 11 attacks in New York and Washington.
Rasmussen's comments about engaging with moderates echoed recent statements by the foreign ministers of France and Britain who have argued that it is time to engage with Taliban willing to renounce violence.
The new secretary-general, who is succeeding Dutch diplomat Jaap de Hoop Scheffer, will also have to help rekindle icy ties with Russia, still seething that a Cold War foe is closing in on its borders, and ensure that thousands of NATO troops are eased out of Kosovo without re-igniting the ethnic tinderbox.
A deft hand will also be required to breathe new life into the NATO-Russia Council, the forum where Moscow and the alliance cooperate and air their many differences, amid deep tensions over the war in Georgia a year ago.
On the diplomatic side, Rasmussen has to earn the trust of the Muslim world after Turkey initially objected to his candidacy following the publication of cartoons of the Prophet Mohammed in the Danish press.
Ankara will insist he make good on a pledge to develop a dialogue with the Muslim world during his four-year tenure as NATO's top civilian official.