Gen David Petraeus has indicated to a Senate committee that the security situation in Afghanistan is "tenuous".
He is before the Senate Armed Services Committee after being nominated by President Obama to lead the war.
Gen Petraeus said he supports the president's plan to begin withdrawing troops in July 2011, but emphasised it is the "beginning of a process".
He also said he would assess the rules of engagement which have come under intense criticism in recent days.
"My sense is that the tough fighting will continue; indeed, it may get more intense in the next few months," he said.
In written answers to the committee, he said insurgents were "resilient and still-confident".
Gen Stanley McChrystal, the former US commander in Afghanistan, was fired last week by President Obama.
The general was dismissed after criticising senior US administration officials in a Rolling Stone magazine profile. He has since announced his retirement from the US Army.
Mounting uneaseGen Petraeus, 57, was nominated by President Obama last week to replace Gen McChrystal in Afghanistan.
The widely-lauded general has formidable political and diplomatic skills. He has been credited with having turned around the military situation in Iraq with a "surge" there.
There is a broad consensus among lawmakers that there is not a better man for the job, the BBC's defence correspondent Nick Childs says.
But his confirmation hearing is also likely to be become a platform for the airing of mounting unease in the Congress over the administration's Afghan strategy, our correspondent says.
Republicans are expected to question Gen Petraeus about whether Mr Obama's strategy of commencing a troop drawdown in July 2011 will hamper his leadership of the war effort.
The leading Republican on the Armed Services Committee, Senator John McCain, has been a vocal critic of setting a date for withdrawal.
Some in Washington political circles also question the reliability of the Afghan government as a partner and the quality of Afghan forces.
The Obama administration is stressing that Gen Petraeus represents continuity and reassurance, and that he is in many ways the father of the strategy to which the US and its allies are wedded.
In December, President Obama ordered 30,000 extra troops into Afghanistan, an announcement that received support from both parties.
