WASHINGTON (NYTIMES) - At the State Department, the normally pulsating hub of executive offices is hushed and virtually empty. At the Pentagon, military missions in some of the world's most troubled places are being run by a defence secretary who has none of his top team in place.

And at departments like Treasury, Commerce and Health and Human Services, many senior posts remain vacant even as the agencies have been handed enormous tasks like remaking the nation's health insurance system.

From the moment he was sworn in, President Donald Trump faced a personnel crisis, starting virtually from scratch in lining up senior leaders for his administration. Seven weeks into the job, he is still hobbled by the slow start, months behind where experts in both parties, even some inside his administration, say he should be.

The lag has left critical power centres in his government devoid of leadership as he struggles to advance policy priorities on issues like health care, taxes, trade and environmental regulation.

Many federal agencies and offices are in states of suspended animation, their career civil servants answering to temporary bosses whose influence and staying power are unclear, and who are sometimes awaiting policy direction from appointees whose arrival may be weeks or months away.

"There's no question this is the slowest transition in decades," said R. Nicholas Burns, a former State Department official who served under presidents of both parties and has been involved in transitions since 1988. "It is a very, very big mistake. The world continues - it doesn't respect transitions."

Trump has insisted that the barren ranks of his government are not a shortcoming but the vanguard of a plan to cut the size of the federal bureaucracy. "A lot of those jobs, I don't want to appoint, because they're unnecessary to have," Trump told Fox News in February. "I say, 'What do all these people do?' You don't need all those jobs."

But the president has not proposed any plan for trimming crucial senior positions, and a White House spokesman Lindsay E. Walters said he planned to fill them eventually.

Trump's personnel problems are rooted in a dysfunctional transition effort that left him without a pool of nominees-in-waiting who had been screened for security and financial problems and were ready to be named on Day 1.

In the weeks since, the problem has been compounded by roadblocks of his own making: a loyalty test that in some cases has eliminated qualified candidates, a five-year lobbying ban that has discouraged some of the most sought-after potential appointees, and a general sense of upheaval at the White House that has repelled many others.

Officials involved in and briefed on the situation described it on the condition of anonymity, unwilling to be quoted disparaging Trump or his administration.

But the numbers paint an unmistakable picture. While Trump has won confirmation of 18 members of his Cabinet, he has not nominated anyone for more than 500 other vital posts and has fallen behind his predecessors in filling the important second- and third-tier positions that carry out most of the government's crucial daily functions.

As of Sunday, he had sent to the Senate 36 nominations for critical positions, about half the 70 that President Barack Obama, who was also criticised for early delays, had sent at the same point in 2009, according to figures compiled by the nonpartisan Centre for Presidential Transition at the Partnership for Public Service.

In the vast majority of cases, Trump's administration has not even begun the lengthy screening process - which can take from several weeks to two months - that nominees must undergo before their confirmations can be considered by the Senate.

According to data obtained by The New York Times, the Office of Government Ethics, the independent agency that conducts financial reviews of every presidential nominee, had received only 63 disclosure reports for prospective Trump administration nominees as of March 5, less than a third of the 228 that Obama's team had submitted by that date in 2009.

At the State Department, both deputy-level jobs remain unfilled, along with the posts of six undersecretaries and 22 assistant secretaries. At the Treasury Department, Trump has yet to name a deputy secretary, general counsel or chief financial officer, or any of the three undersecretaries and nine assistant secretaries.

At the Department of Homeland Security, one of three agencies for which the president has nominated a deputy, he has yet to name any of the four undersecretaries, three assistant secretaries or other crucial players like a chief of Citizenship and Immigration Services or a commissioner of Customs and Border Protection.

The staffing delays appear to reflect Trump's lack of experience in government and his deep suspicion of anyone with such a background - two significant factors in his flawed transition.

Another challenge has been the president's attempt to apply to the sprawling federal bureaucracy the same mode of leadership he has used for decades in his business, where he placed exclusive trust in a small, insular team.

"The approach that the president took as a businessman and a candidate is simply not scalable to the challenge of filling out the rest of the government leadership," said Max Stier, the executive director of the Center for Presidential Transition.

At the Pentagon, Defence Secretary Jim Mattis is overseeing missions in Iraq, Afghanistan and Yemen without his own leadership team.

Some nominees, including billionaire Vincent Viola, Trump's choice for Army secretary, had to withdraw because background checks - which would normally have been completed weeks earlier - revealed insurmountable problems, like financial entanglements. That has left Mattis to rely on holdovers like Robert O. Work, the deputy defense secretary in Obama's administration, and senior civil servants.

"He is really missing three to four levels of his leadership team," said Michèle Flournoy, the undersecretary of defence for policy under Obama, who said she had removed herself from consideration to be Mattis' deputy because she did not agree with the new administration's values and policy direction.

"The White House personnel system has really put an emphasis on loyalty to Trump, and they have ruled out anyone who said anything bad about him."

At the Environmental Protection Agency (EPA), Scott Pruitt, the administrator, was confirmed by the Senate last month, and he has hired a chief of staff and a few others. But the White House has yet to nominate anyone to fill another dozen critical jobs requiring Senate confirmation, like the assistant administrators who oversee clean air regulation and water protection.

Those offices sit empty even as Trump says he wants the EPA to start rolling back several major Obama-era regulations, including a rule on water pollution.

"It will be impossible for them to carry out that agenda unless they can get people in place," said Jeffrey Holmstead, an assistant EPA administrator in Bush's administration.

Trump also faces the task of appointing US attorneys around the country after asking 46 to submit their resignations on Friday.