President-elect Joe Biden on Monday accused President Trump and his political appointees of obstructing the transition of power to his incoming administration, particularly in the national security sphere, an escalation in tone after reports of isolated difficulties in the transition process last week.

Biden specifically called out the Office of Management and Budget and the Defense Department as agencies where his transition team had encountered “roadblocks” from political leadership.

“Right now, we just aren’t getting all the information that we need from the outgoing administration in key national security areas. It’s nothing short, in my view, of irresponsibility,” Biden said of the resistance his team was facing. He warned that such delays could allow enemies of the United States to take advantage of vulnerabilities, citing a recent massive cybersecurity breach that compromised several U.S. agencies.

“As our nation is in a period of transition, we need to make sure that nothing is lost in the handoff between administrations. My team needs a clear picture of our force posture around the world and our operations to deter our enemies,” Biden said in remarks from Wilmington, Del. “We need full visibility into the budget planning underway at the Defense Department and other agencies in order to avoid any window of confusion or catch-up that our adversaries may try to exploit.”

The pointed accusation by a president-elect that the incumbent was putting national security at risk by refusing to cooperate underscored the unprecedented and divisive nature of the current transition.

With less than a month before Inauguration Day, Biden has been laying the groundwork for how to tackle the gargantuan twin challenges that he will face as soon as he assumes the presidency — ending the coronavirus pandemic and rebuilding the economy. He has steadily filled vacancies in his Cabinet and in senior positions in his government, which will take over at noon on Jan. 20.

He has been met with remarkable resistance from Trump, who has refused to concede the election and has continued attempting to overturn the results. Trump blocked any transition efforts outright for more than two weeks before relenting, at least initially. He has declined to say whether he will attend Biden’s inauguration, and the incoming team planning the event assumes he will not.

Trump also has appeared increasingly uninterested in the nation’s most critical matters, instead pressuring allies to change the results of an election that his own administration said had been free of widespread fraud. On Dec. 23, he vetoed a defense authorization bill that included raises for service members, forcing a veto override effort this week. He complained that the measure would allow the renaming of military facilities honoring Confederate soldiers and had not included an unrelated measure punishing social media companies. He threatened to veto a coronavirus relief measure, delaying benefits before he eventually signed it Sunday.

On Monday, Trump continued to tweet conspiracy theories about the election — at one point retweeting a view that opponents were guilty of “treason” — and spent much of the day at his private golf club in Florida.

Meanwhile, Biden and Vice President-elect Kamala D. Harris attended a virtual briefing with 15 national security and foreign policy advisers, including several would-be Cabinet nominees. In comments that followed, Biden said the advisers told him that many of the agencies critical to national security have sustained “enormous damage” during the Trump administration.

“Many of them have been hollowed out in personnel, capacity and in morale, in the policy processes that have atrophied or have been sidelined, in the disrepair of our alliances . . . in the general disengagement from the world,” Biden said. “And all of that makes it harder for our government to protect the American people, to defend our vital interests in a world where threats are constantly evolving and our adversaries are constantly adapting.”

Acting defense secretary Christopher Miller disputed Biden’s accusation, saying in a statement Monday night that more than 400 defense officials have participated in 164 meetings with the transition team and provided more than 5,000 pages of documents. Miller said these efforts “already surpass those of recent administrations,” despite a compressed time frame.

Miller had, however, abruptly postponed all transition meetings on Dec. 18, saying in a statement then that the Biden team and Trump administration had mutually agreed on a pause through the holiday season. Biden team officials have denied that was the case. An official familiar with the situation who spoke on the condition of anonymity to discuss the matter said no meetings have occurred since then. The Defense Department announced late Monday that three meetings are scheduled for this week, two related to the pandemic and one on cybersecurity.

The Office of Management and Budget did not immediately respond to Biden’s comments.

Biden said Monday that most government agencies have shown “exemplary cooperation” with his transition team, especially given the challenges of the pandemic and the Trump administration’s effort to stall conversations, but that his staffers have encountered “obstruction from political leadership” when they could not afford to waste any time. He noted that four years ago, he and then-President Barack Obama gave the incoming Trump-Pence administration “access to all that we had.”

In raising concerns about the transition, Biden was careful to distinguish between political appointees in the agencies and the career professionals who he said had cooperated fully.

“They never stopped doing their job and continued to serve our country, day in and day out, to keep their fellow Americans safe,” Biden said of the career government workers. “These agencies are filled with patriots who’ve earned our respect, and who should never be treated as political footballs.”

As an example of the potential impact of the obstruction, Biden pointed to the pandemic, which in December killed more Americans than in any previous month. More than 330,000 have died since March, with nearly 19.3 million sickened.

“We’ve learned so painfully this year the cost of being unprepared,” Biden said.

Biden said that under his administration, the Federal Emergency Management Agency will play an “enormous part” in the “safe, equitable and efficient distribution of vaccines to as many Americans as possible, as quickly as possible.” Harris and her husband plan to receive vaccinations in Washington on Tuesday.

“We want to make sure that our administration is poised to make full use of FEMA’s domestic reach and capacity,” Biden said.

In his Monday remarks, Biden also took issue with Trump’s handling of foreign affairs, repeatedly saying that the United States needs to strengthen its alliances with like-minded countries, not just to confront the pandemic but also to address climate change and “strategic challenges” from China and Russia.

“Right now there’s an enormous vacuum,” Biden said. “We’re going to have to regain the trust and confidence of a world that has begun to find ways to work around us or work without us.”

Biden said he was also briefed Monday on the steps needed to “clean up the humanitarian disaster that the Trump administration has systematically created on our southern border.”

Biden said that the work will start on his first day in office but could take some time, especially when it comes to rebuilding the nation’s capacity for processing asylum claims. He has previously pledged a comprehensive immigration plan on his first day in office.

“We’re going to work purposefully, diligently and responsibly to roll back Trump’s restrictions starting on day one,” he said. “But it’s not as simple as throwing a switch to turn everything back on, especially amid a pandemic.”

Biden opened his comments by addressing the Christmas Day explosion in Nashville, saying federal, state and local law enforcement “are working around-the-clock to gain more information on motive or intent.” He praised Nashville police and other first responders, saying that “their bravery and coolheadedness” probably saved lives. Local and federal authorities have said a local man whose remains were found in the wreckage was responsible for the explosion, which spread destruction for blocks.

“This bombing was a reminder of the destructive power that individuals and small groups can muster,” Biden said, “and the need for continuing vigilance across the board.”