Speaking during an interview taped for NBC's "Meet the Press," Petraeus said Obama asked him for his "best military advice," and his job is not to be political about war.
If sufficient progress has not been made by the president's deadline to start withdrawal, "I would communicate that to him. ... That's real life," said the general who succeeded the ousted Gen. Stanley McChrystal.
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.
Petraeus and other military officials have warned of more combat casualties as additional U.S. troops are sent to the fight. Last fall, 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. Petraeus conceded the U.S. mission in the war-weary South Asian nation is tough, and will remain so.
Sen. Jack Reed, D-R.I., didn't disagree.
"I believe that you'll see people moving out, missions changing, but I don't sense immediately a dramatic pell mell withdrawal. I think you'll see a change based on conditions," he said on "Fox News Sunday."
Reed, a member of the Senate Foreign Relations Committee, said much of the drawdown will be dependent on the ability of the Afghan national army to get trained and provide their own security. He added that the results are mixed right now, but progress is being made.
"That is probably related in part to their knowing there is a deadline for them to step up to the plate," he said.
Even with success in Afghanistan, the country is unlikely to become an industrialized democracy, Petraeus said. A much more modest goal is to enhance the authority of the central government in Kabul and prevent influence or worse from terror groups like Al Qaeda and the Taliban.
Petraeus said arresting Al Qaeda leader Usama bin Laden remains a primary goal, and the loss of faith among Taliban in their detached leadership is helpful.
"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.
As for Petraeus' apolitical stance, that applies to his own ambitions. The general said he's not a politician and never will be one.
The popular Army four-star general, who took charge of the war in Afghanistan in July, has been rumored to covet the Oval Office. But he said in the recorded interview that he has no desire to run for president.
Petraeus, who's been credited with a successful war strategy in Iraq, said he can dismiss "with absolute conviction" the idea that he could become a politician. And as for trying to become commander in chief, he says "no way, no how."
The Associated Press contributed to this report.