The top commander in Afghanistan said Thursday he prefers a robust U.S. combat force of 68,000 in 2013, signaling a potential halt in the drawdown and complicating any effort by President Barack Obama to accelerate the timetable after more than a decade of war.

"My opinion is that we will need significant combat power in 2013," Marine Gen. John Allen told the Senate Armed Services Committee.

Pressed by the panel's top Republican, Sen. John McCain, on whether a force of 68,000 would be sufficient, Allen said, "Sixty-eight thousand is a good going-in number, but I owe the president some analysis on that."

In his second public hearing on Capitol Hill this week, the commanding general of the war in Afghanistan argued that the war can still be won, and he warned that a withdrawal of coalition forces would create a vacuum in the region.

"Should the U.S. leave Afghanistan, should ISAF, NATO leave Afghanistan, that would create in my mind, for all intents and purposes, a geopolitical vacuum," Allen said, as the Afghan National Security Forces prepare to take over responsibility.

Allen's comments represent a marker for the military. Obama faces increasing political and public pressure to accelerate the timetable after more than 10 years of fighting and recent incidents that dealt a major setback to the fragile U.S.-Afghanistan relationship. Afghan outrage over the burnings of Qurans and a shooting spree that left 17 Afghan civilians dead have been blamed on Americans.

Sen. Susan Collins, R-Maine, tried to get Allen to admit how much money the U.S. will have to pay to fund the Afghan National Security Forces over the next decade, but Allen said he could not say. He did acknowledge that the current annual cost to fund the ANSF's 352,000 troops is between $4 billion and $5 billion, but the entire annual revenue for Afghanistan is only $2 billion, so the international community will be helping the Afghans for some time.

Allen said he remains optimistic that the war can still be won.

"If I didn't think it was do-able I would tell you, and I would tell you very quickly because I wouldn't want to spend another life in this fight if it wasn't do-able," Allen said.

Asked later by Sen. Lindsey Graham whether the current strategy in Afghanistan represents "the last best chance for success in Afghanistan," Allen said quietly, "I concur with that, senator."

NBC News' Courtney Kube and The Associated Press contributed to this report.

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The top U.S. commander for Afghanistan told lawmakers on Tuesday that an investigation is under way into the command climate surrounding the sergeant accused in the shooting massacre of 16 Afghan civilians. This is separate from the criminal investigation into the killings, allegedly committed by Army Staff Sgt. Robert Bales.

Gen. John Allen, in testimony before the House Armed Services Committee, revealed the investigation will look into the command and control process, how and why Bales was assigned to the Special Operations Village stability mission and what the combat relationships were within that unit. 

It's expected the investigation will also look into the presence of alcohol on that small outpost in southern Afghanistan and whether there was a breakdown in command leadership in that unit.

Because of the criminal investigation under way, Allen offered no additional information on the suspect, other than to repeat that the U.S. military is committed to seeking justice in the case.

Bales' lawyer questions evidence in Afghan killings

Bales, 38, of Lake Tapps, Wash., has not been charged yet in the March 11 shooting spree. He was deployed to Afghanistan in December.

U.S. military officials said they still expect him to be charged later this week. The charges will be released by the U.S. military in Afghanistan, they said.

NBC News Pentagon Producer Courtney Kube contributed to this report.

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