"I won't risk your lives unless it is absolutely necessary," Obama told service men and women at Naval Air Station Jacksonville. He promised a "clear mission" with defined goals and the equipment needed to get the job done.
Obama, who is in the process of weighing options put forward by the Pentagon that include various levels of increased troops, spoke of the latest example of the dangers and sacrifices there -- helicopter crashes that killed 14 Americans in the deadliest day for the U.S. mission in Afghanistan in more than four years.
"Fourteen Americans gave their lives. And our prayers are with these service members, their civilian colleagues and the families who loved them," Obama said. "They were willing to risk their lives, in this case to prevent Afghanistan from again becoming a safe haven for Al Qaeda and its extremist allies."
His visit to the naval air station came after he met at the White House with his national security team for a sixth full-scale conference on the future of the faltering war.
The administration is debating whether to send tens of thousands more troops to the country, while the Afghan government is moving to hold a Nov. 7 runoff election between President Hamid Karzai and challenger Abdullah Abdullah. The runoff comes after complaints by international monitors of fraudulent voting in the first election.
But, Obama's critics say the time for him to make a decision is running out.
"Republicans want very much to support the president's decision," Senate Republican Whip Jon Kyl, R-Ariz., told "Fox News Sunday." But he cited Gen. Stanley McChrystal's own warning that U.S. and NATO forces may only have about one year before the insurgency's momentum becomes irreversible.
"It's been more than two months since the recommendation went to the president. And Gen. McChrystal is talking about a 12-month time frame," Kyl said. "So clearly time is of the essence here."
Sen. John McCain, R-Ariz., also called for an end to the delay.
"The sooner the decision is made, the sooner we get people over there and are able to implement the strategy that will succeed," he told CBS' "Face the Nation." "Every day we delay will be a delay in this strategy succeeding."
Waves of boisterous cheers greeted the president in Florida. Obama noted that representatives of all the nation's military services attended the gathering.
Obama did not tip his hand on how he might decide.
"I will never rush the solemn decision of sending you into harm's way," he said.
If it is necessary, Obama added, "we will back you up to the hilt."
The Associated Press contributed to this report.