The man chosen to take charge of the US military in Afghanistan, Gen David Petraeus, has warned of an escalation of violence in the coming months.
"The going inevitably gets tougher before it gets easier," he told the Senate Armed Services Committee, which backed his nomination to lead the war.
The general said troops were engaged in a contest of wills with the Taliban and promised a more co-ordinated approach.
President Obama chose Gen Petraeus after sacking Gen Stanley McChrystal.
The outgoing general and his aides criticised senior administration officials in a Rolling Stone magazine article. He has since announced his retirement.
Rules of engagement
In written answers to the Senate committee on Tuesday, Gen Petraeus described the security situation in Afghanistan as "tenuous" and insurgents as "resilient and still-confident", particularly in the south of the country.
However, he did say that he believed progress was possible.
"They can sense concern in various capitals around the world and of course they want to increase that concern," he said.
"My sense is that the tough fighting will continue; indeed, it may get more intense in the next few months," he added. "As we take away the enemy's safe havens and reduce the enemy's freedom of action, the insurgents will fight back."
The general said he supported the president's plan to begin withdrawing troops in July 2011, but emphasised that there would be "certain tweaks, refinements, perhaps significant changes" after the White House's year-end review.
Senator John McCain said the US could not "afford to have a stay-the-course approach to starting our withdrawal in July 2011 when the facts on the ground are suggesting that we need more time".
Later, Gen Petraeus warned that raising the standards of the Afghan army and police was a "hugely challenging" task, he said, comparing it to "building an advanced aircraft while it is in flight, while it is being designed and while it is being shot at".
He also said he would look very hard at the current rules of engagement for US ground and air forces, which were drawn up to reduce civilian deaths but have been criticised for putting units at unnecessary risk.
"Those on the ground must have all the support they need when they are in a tough situation," he told the committee.
Gen Petraeus, 57, was nominated by President Obama last week to replace Gen McChrystal as commander of US and Nato forces in Afghanistan.
The widely-lauded soldier 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.
Meanwhile, Defence Secretary Robert Gates held talks at the Pentagon with his UK counterpart, Dr Liam Fox.
During their meeting, Dr Fox stressed the need to keep focus on ensuring that the necessary security conditions are met before transition to the Afghan authorities can take place.
"We cannot afford Afghanistan to lapse back into a failed state, which will create a security vacuum, contaminate the region and threaten the national security of the UK and its allies. That is why we are there and that is why we stay," he said.