WASHINGTON -- The true test now begins for Donald Trump.

The Republican president-elect paid little attention to transition planning leading up to his stunning victory. With 72 days before he takes control of the executive branch, Trump and his senior team on Wednesday immediately began the herculean task of picking a Cabinet and tapping hundreds of appointees to senior roles in key departments -- State, Defence, Homeland Security, Commerce and Treasury among them -- many requiring multiple security reviews or Senate confirmation.

"They have a long way to go," said Max Stier, president and CEO of the Partnership for Public Service, an outside group that was working with both campaigns on transition planning since the summer. "It's imperative to have the right people brought in fast and they're prepared."

Stier described the transition as "a point of maximum vulnerability" for the nation.

As president-elect, Trump is entitled to get the same daily intelligence briefing as President Barack Obama -- one that includes information on U.S. covert operations, information gleaned about world leaders and other data gathered by America's 17 intelligence agencies.

Trump's senior team huddled privately on Wednesday to begin a more focused period of transition planning. The group included the transition chairman, New Jersey Gov. Chris Christie, Republican National Committee Chairman Reince Priebus, Alabama Sen. Jeff Sessions, Vice-President-elect Mike Pence and daughter Ivanka Trump's husband, Jared Kushner, among others.

The team is putting a premium on quickly filling key national security posts, according to people familiar with the conversations but not authorized to discuss them publicly.

"We had a great meeting and I thought it was valuable. And lots of work has been done," said Sessions. "So it's the beginning, really a solid beginning. First phase of the beginning. "

Trump is eyeing a senior team drawing from a small group of high-profile supporters, including Priebus, Christie, former New York Mayor Rudy Giuliani, former House Speaker Newt Gingrich and retired Lt. Gen. Michael Flynn. Yet Bill Hagerty, Trump's director of presidential appointments, declined to detail a timeline for Trump's first personnel moves.

A chief of staff is traditionally appointed in the initial weeks after an election.

The White House, which had been planning for the transition long before Election Day, said Trump's team would start working directly this week with agencies across the federal government while getting detailed briefings that include budget materials and organizational flow charts. The White House said the Obama administration would also host two "exercises" involving multiple agencies to help familiarize Trump's team with government systems for responding to domestic emergencies.

A small Trump transition team has been meeting since early August to discuss legislative priorities and plans for taking over agencies. While Christie provided Trump with weekly updates, until now, the campaign and transition operations functioned as relatively distinct entities and in different cities -- Trump's campaign in New York and the transition team in Washington.

An organizational chart for the transition team obtained by The Associated Press confirms that some familiar names are playing senior roles in shaping a Trump administration.

National security planning was being led by former Michigan Rep. Mike Rogers, who previously worked for the FBI. Joseph "Keith" Kellogg, a retired Army lieutenant general, was heading defence planning. For domestic issues, the Trump transition team was relying on the leadership of Ken Blackwell, a former Cincinnati mayor and Ohio secretary of state.

A Trump spokesman did not respond to questions about the team, but key allies rejected the notion he was behind where he should be in planning.

"Donald Trump is taking this very seriously," Priebus said, suggesting that Trump's deal-making skills would enable him to quickly "make things happen for the American people."

It's far from clear who would occupy Trump's Cabinet and senior staff. His inner circle is famously small, defined by loyalty to the president-elect and largely devoid of establishment leaders.

Trump offered a roadmap for prospective administration figures while on stage during his victory speech early Wednesday.

The president-elect praised Christie, despite the New Jersey governor's abysmal poll numbers at home and continued scrutiny for the so-called Bridgegate scandal.

Giuliani emerged as one of Trump's closest advisers and regular travelling partners in the campaign's final stretch. The former federal prosecutor is a possible fit for a post like attorney general or national security adviser.

Sessions was the first major Capitol Hill lawmaker to back Trump and his chief of staff, Rick Dearborn, has emerged as a key figure in transition planning. While many Republican national security leaders shunned Trump, Flynn, a retired lieutenant general, was the Republican nominee's most experienced national security voice.

Priebus is being considered for a top post, including chief of staff, while Gingrich could emerge as a possible secretary of state. Tennessee Sen. Bob Corker, who chairs the Foreign Relations Committee, may also be in line for a job. Steven Mnuchin, a Goldman Sachs veteran and CEO of a private investment firm, served as Trump's finance chairman and instantly becomes a contender for Treasury secretary.