The White House has given approval for Joe Biden to receive the president's daily brief, a collection of classified intelligence reports prepared for the US leader.
It means Mr Biden, as president-elect, will have access to the latest top-secret information about major security threats around the world.
The move usually happens swiftly after an election but had been held up for weeks as Mr Trump declined to concede.
Allowing access for Mr Biden marked a milestone on his way to taking over from Mr Trump on Jan 20.
The president declined to answer any questions, including whether he would concede defeat.
Meanwhile, it emerged that renovations of living areas are underway for Mr Trump, and his wife Melania, at Mar-a Lago, his club in Palm Beach, Florida.
Secret Service agents in the president's protective detail have also been quietly asked if they want to relocate there.
Former presidents receive Secret Service protection for life, and the agency's Miami office will look at whether further security infrastructure is required at the resort.
Mr Trump changed his permanent residence from New York to Florida last year.
Lois Frankel, the Democrat congresswoman who represents Palm Beach, said it would seek reimbursements from the US government for any local police costs incurred by having Mr Trump living there.
Mr Trump is expected to be at Mar-a-Lago by Jan 20, when Mr Biden takes over as president.
The president indicated he would not concede before leaving.
He reposted a series of tweets by Randy Quaid, the actor, who is calling for another election. Mr Quaid said there should be an "in-person-only-paper ballot re-vote".
Mr Trump also reposted a message from a supporter, saying "I concede NOTHING!!!!!"
A new poll showed Mr Trump was the clear favourite for the Republican presidential nomination in 2024.
The Politico/Morning Consult poll had him on 53 per cent among Republicans. Mike Pence, the vice president, was in second place on 12 per cent, and Donald Trump Jr. third on eight per cent.
Mr Trump's efforts to overturn the 2020 election results received a further blow when Pennsylvania officially certified that Mr Biden had won there.
Mr Biden pressed ahead at speed with the transition, which formally began on Monday after Emily Murphy, head of the General Services Administration, formally ascertained that he was the apparent winner.
It frees up $7.3 million in government funds for Mr Biden to set up a transition office, and his aides began communicating with dozens of government agencies to get up to speed.
That included the Centres for Diseases Control and Prevention, and the Trump administration's Warp Speed project. The Biden team will seek information in plans for distributing vaccines, and data on the pandemic.
The Pentagon said it had established a transition task force and was "in discussions on the next steps" with the Biden team. Mr Biden was expected to receive access to intelligence briefings imminently. His officials will begin moving to government email addresses.
Mark Meadows, Mr Trump's chief of staff, sent an email to officials there saying the administration would "comply with all actions needed to ensure the smooth transfer of power."
Unveiling some of his key cabinet and national security picks to the public in Delaware, Mr Biden said they were "experienced, crisis-tested leaders who are ready to hit the ground running on day one."
They included Tony Blinken as secretary of state, John Kerry as climate tsar, and Jake Sullivan as national security adviser.
All are veterans of the Barack Obama administration, and it signaled a rapid dismantling of Mr Trump's "America First" policies.
Mr Biden said he had now received "18 or 20" calls from world leaders. He said: "I've been stuck by how much they're looking forward to the US reasserting its role as a global leader."
The president-elect said his team would "reimagine American foreign policy and national security for the next generation."
But Marco Rubio, the Republican senator said: "Biden's cabinet picks went to Ivy League schools, have strong resumes, attend all the right conferences, and will be polite and orderly caretakers of America's decline."
Another Republican called them "delightfully boring" careerists.
Internal tensions also emerged as some long-serving members of Mr Biden's campaign team suggested they had been "cut out: in favour of officials from the Obama era.