WASHINGTON -- Progress in Afghanistan only began this spring and needs time to take root, Army Gen. David Petraeus said in comments broadcast Sunday that were aimed at shoring up American support for the war.

Petraeus, who's been credited with a successful war strategy in Iraq and who took charge of U.S. and NATO military operations in Afghanistan in July, described an "up and down process" of seizing Taliban-controlled territory and creating "small pockets of progress" that he hoped will expand.

The goal, he told NBC's "Meet the Press," is to keep al-Qaida and other extremist groups at bay while the Afghan government has a chance to take control and earn the trust of the local population.

"We're here so that Afghanistan does not once again become a sanctuary for transnational extremists the way it was when al-Qaida planned the 9/11 attacks in the Kandahar area," Petraeus said in an interview taped in Kabul, the Afghan capital.

The general also stressed that the withdrawl of U.S. troops scheduled to begin in July 2011 would be based on conditions on the ground, and that he could advise President Obama to keep U.S. forces in Afghanistan if he felt it was not the right time for a drawdown.

"What the president very much wants from me and what we talked about in the Oval Office is the responsibility of a military commander on the ground to provide his best professional military advice. Leave the politics to him," Petraeus told host David Gregory. "Certainly, I am aware of the context in which I offer that advice but that just informs the advice, it doesn't drive it. The situation on the ground drives it."

Petraeus' comments come as U.S. support for the 9-year war is slipping and the death toll is climbing. July was the deadliest month for U.S. forces, when 66 troops were killed.

On Sunday, Rethink Afghanistan released a video hammering Petraeus' comments as just spin:

Petraeus and other military officials have warned of more combat casualties as additional U.S. troops are sent to the fight. Last fall, President Barack Obama authorized 100,000 troops in Afghanistan - triple the level from 2008.

Obama's Democratic supporters have reluctantly swung behind the plan, but lawmakers are beginning to question whether Afghanistan can be won.

As the fighting intensifies, the Pentagon and White House are hoping that political support for the war can hold at least through year's end to give Petraeus time to show progress. Petraeus is expected to give an updated assessment to Congress in December.

Petraeus said in the interview that the war only recently has been given the right "inputs," or resources: more U.S. and Afghan troops to take over Taliban territory and more civilians to restore services to the population.

"There is understandable concern and, (in) some cases, frustration," Petraeus said. "Therefore we have got to really put our shoulders to the wheel and show during the course of this year that progress can be achieved."

Petraeus described Afghanistan as a tough and enduring fight that would require its "character and its size being scaled down over the years." If the U.S. loses, there would likely be a bloody civil war followed by a takeover by extremists. If the U.S. succeeds and Afghanistan stabilizes, the country could become the region's new "Silk Road" with the potential to extract trillions of dollars worth of minerals, he said.

But the goal is not to turn Afghanistan into an industrialized democracy, he said. Even if the nation relies heavily on tribal councils for governance, the central government in Kabul could still run the nation effectively without influence from extremist groups such as al-Qaida.

Petraeus said arresting al-Qaida leader Osama bin Laden remains a primary goal.

He also said the Taliban leadership had detached itself from much of the fighting, occasionally sending messages via cell phones, but is not as engaged in the war as before.

"We actually see discussions among (Taliban foot soldiers), chatter among them ... wondering where their senior leaders are, and wondering why (Taliban leader) Mullah Omar hasn't set foot back in Afghanistan or even been heard from now in months and months and months," he said.

When asked about the rocky U.S. relationship with Afghan President Hamid Karzai, Petraeus denied there were serious problems and defended Karzai as a leader trying to curb corruption. Petraeus said he and Karzai usually talk once a day, sometimes more, and take walks in the garden behind Karzai's house.

"We have the kind of relationship that, I believe, we can each be forthright with the other and that means occasionally, again, confronting issues that are difficult for either of us," he said.