Mullen said that he did not know how many more troops would be requested by General Stanley McChrystal, the commanding general of US and Nato troops in Afghanistan, in the next few weeks.
The senate panel hearing is to assess the nomination of Mullen for a second term as Obama's senior military adviser.
While Mullen said that extra troops would likely be needed in Afghanistan, Carl Levin, a Democrat senator who chairs the panel, said that the US should aim to support Afghan security forces so that they can work independently.
"We will need resources matched to the strategy, civilian expertise matched to military capabilities, and the continued support of the American people"
Admiral Michael Mullen, chairman of the US joint chiefs of staff
"Providing the resources needed for the Afghan army and Afghan police to become self-sufficient would demonstrate our commitment to the success of a mission that is in our national security interest, while avoiding the risks associated with a further increase in US ground combat troops," Levin said.
Levin and several other leading Democrats have recently said that sending extra US troops to Afghanistan may be counter-productive in the fight against the Taliban.
Some Democrats have said too large a US military presence will be viewed by Afghans as an occupation force, while Levin wants the number of trained Afghan security forces to be increased.
But Senator John McCain, the senior Republican on the panel, said that committing too few forces to Afghanistan could undermine the efforts made so far.
Senator Joseph Lieberman, an independent, said that Afghans would get the wrong message if extra troops are not sent.
"They're essentially going to decide we're on our way out," Lieberman said.
About 65,000 troops are in Afghanistan, while thousands of trainers are due to visit the country by the end of the year to assist Afghan security personnel.
The war in Afghanistan is losing support among Americans, with 58 per cent of the US public now against the military campaign, according to a CNN/Opinion Research poll released on Monday.
Lawrence Korb, a former assistant secretary of defence who was an adviser to Obama during his presidential campaign, said the president had to decide whether to try to convince a sceptical American public that more forces should go to Afghanistan.
"More US troops should provide more security, which will give you an opportunity to convince the people that they do not need to throw in their lot with the Taliban. You are fighting for what the US military would call the 'hearts and minds' of the people in Afghanistan," he told Al Jazeera.
"That is the key - are the [Afghan people] convinced enough that the US and Nato forces can provide the security so that they don't have to fear the Taliban?"
Mullen said he believed that the war in Afghanistan can be won, and called for the support of the American people.
"We can get there. We can accomplish the mission we've been assigned," Mullen said.
"But we will need resources matched to the strategy, civilian expertise matched to military capabilities, and the continued support of the American people."