Biden's team is pressing ahead with a transition despite President Donald Trump's refusal to concede the Nov. 3 election, which major news organizations called for Biden on Nov. 7.
Psaki said the fact that Biden was not yet receiving classified intelligence briefings could hurt his preparations to govern.
“It's been six days, but with every day that passes on, it becomes more concerning that our national security team and the president-elect and the vice president-elect don't have access to those threat assessments, intelligence briefings, real time information about our engagements around the world," Psaki said on a call with reporters.
"You don’t know what you don’t know.”
Psaki cited the 9/11 commission report that listed a delayed transition after the 2000 election as a reason officials were not well prepared for the attacks on the United States on Sept. 11, 2001.
A handful of Republican senators have urged the Trump administration to allow Biden to receive presidential daily intelligence briefings, which the president-elect traditionally receives before taking office.
Psaki did not name the national security experts who would be briefing the former vice president.
Biden this week formed a task force of independent health experts who are advising him on his plans to fight COVID-19.
The General Services Administration, the federal agency that releases funding to an incoming president, has not yet recognized Biden's victory, denying him access to federal office space and resources.
Psaki called on the agency's Trump-appointed administrator, Emily Murphy, to sign off on the transition, but said Biden's team was still hoping it would not have to take legal action to make it happen.
Another transition official, Yohannes Abraham, said they were prepared to conduct a transition without the agency's recognition if necessary.
Biden this week held discussions with transition advisers at his homes in Wilmington and Rehoboth, Delaware, on possible Cabinet picks. Psaki declined to comment on specific names under consideration.
Biden has named long-serving aide Ron Klain as his White House chief of staff.