WASHINGTON - U.S. President-elect Joe Biden's transition to power in Washington has formally started after a government agency declared him the apparent winner of the 2020 presidential election, even as President Donald Trump continues his long-shot attempt to upend Biden's victory at the polls.

Biden's team of advisers immediately started reaching out Monday night to Trump officials throughout the government to learn about possible national security threats the country faces, and other immediate issues Biden will face when he is inaugurated January 20.

Pentagon officials said Biden transition team members contacted the Defense Department soon after Emily Murphy, the administrator of the General Services Administration, determined that Biden is the "apparent" election winner and that the transition can begin. Murphy's action allows Biden to tap public funds for the transition, to receive security briefings and his transition officials to gain access to federal agencies.

What Resources Does the Biden Transition Get? Federal law provides for certain activities to ensure a smooth transition of power

It was not immediately clear when Biden would receive his first classified national security briefing as president-elect. Biden has discussed security issues with his team of intelligence and military advisers but has yet to be handed the President's Daily Brief, the U.S. intelligence community's assessment of worldwide threats.

Of immediate concern for Biden is the fight to control the surging number of coronavirus cases in the U.S. Biden officials want information on the Trump administration's timetable to approve three vaccines against the virus in the next few weeks and plans for widespread inoculations of Americans starting before the end of the year and extending well into 2021.

Health officials say approval of the vaccines by the Food and Drug Administration could prove to be more uneventful than the difficult task of distributing the vaccines throughout the country and scheduling millions of people to get the shots. Polls show about four in 10 Americans are wary about getting vaccinated or have already decided against it, potentially making it more difficult to control the pandemic.

Other Biden transition teams are contacting officials at numerous agencies throughout the government to verse themselves on pending policy issues as well as to learn the extent to which the Trump administration removed staff members over the last four years to get rid of what the president deemed to be an entrenched "Deep State" at odds with his view of a limited government.

Meanwhile, at his transition base in his hometown of Wilmington, Delaware, Biden introduced some of the top appointees he named Monday, including Antony Blinken as secretary of State, Avril Haines as director of national intelligence and Alejandro Mayorkas as Homeland Security chief.

Trump reluctantly eased the path for the orderly Biden transition to power to start by acquiescing in Murphy's declaration that Biden was the apparent winner of the contentious, months-long campaign.

Trump said the transition was "in the best interest of our country."

But the president vowed to continue his fight against the election outcome, saying, "Our case STRONGLY continues, we will keep up the good ... fight, and I believe we will prevail!"

Trump offered no concession to Biden and has not called him with congratulations, even as numerous world leaders have offered good wishes to the prospective 46th U.S. president.

Trump's defeat makes him the fifth U.S. president in the country's 244-year history to lose re-election after a single four-year term in the White House.

Trump has lost more than 30 legal challenges alleging vote and vote-counting irregularities in key battleground states but did not acknowledge defeat in allowing the Biden transition to power to move forward. Trump is continuing several lawsuits or appeals of cases he has lost to try to overturn Biden's victory.

On Tuesday morning, Trump said on Twitter, "Remember, the GSA has been terrific, and Emily Murphy has done a great job, but the GSA does not determine who the next President of the United States will be."