Donald Trump is in no hurry to find permanent replacements for the quarter of his cabinet currently serving in an acting capacity, because, the president said on Sunday, the situation gives him “more flexibility”.

His words will do little to dampen rising concern about the lack of experienced counsel around a president who said on Sunday he might soon react to the government shutdown by declaring a national emergency, a move likely to lead to a collision with a Congress part-controlled by Democrats.

“I am in no hurry,” Trump told reporters as he left for Camp David and meetings on the shutdown. About a quarter of the US government has been closed for more than two weeks and about 800,000 workers are without pay.

“I like acting,” Trump said. “It gives me more flexibility. Do you understand that? I like acting. So we have a few that are acting. We have a great, great cabinet.”

He did not elaborate on why acting officials give him more flexibility or say why that should be a good thing, but he spoke as critics continued to voice alarm over the departure of a succession of senior advisers who were seen as able to restrain his wilder impulses.

Secretary of state Rex Tillerson, defense secretary Jim Mattis, national security adviser HR McMaster, attorney general Jeff Sessions and chief of staff John Kelly have all been forced out by Trump. United Nations ambassador Nikki Haley resigned. Mattis, Sessions, Haley and Kelly’s replacements are all serving in an acting capacity.

On Saturday night, the Pentagon chief of staff – not a cabinet post – announced his resignation. Obama-era UN ambassador Samantha Power responded by tweeting: “It is [5 January] 2019: we have no Secretary of Defense & now no DoD chief of staff, no Attorney General & no evident guardrails in Trump’s inner circle.

“The President is unhinged – even GOP zealots know it. Rs inside & outside the admin must stop enabling Trump & protect America.”

Adam Schiff, chair of the House intelligence committee, told CNN on Sunday he agreed “that all of the adults are one by one being forced out of the room. Anyone who had the standing … to tell the president what he needed to hear, not what he wanted to hear, has been pushed aside.”

There are 24 cabinet-level positions and six are now filled by individuals serving in an acting capacity, after the departure of members confirmed by the Senate. Other than the White House chief of staff and the vice-president, cabinet-level officials require such confirmation. Federal law allows presidents to temporarily fill vacancies by appointing acting department heads who are already employees there.

Acting defense secretary Patrick Shanahan listens as Trump speaks during a cabinet meeting on Thursday. Photograph: REX/Shutterstock

Trump said last month he was in no hurry to name a new defense secretary, after Mattis quit. On Friday he reacted to reports that he was considering the Vietnam veteran and former Democratic senator and presidential candidate Jim Webb by praising the acting secretary, Patrick Shanahan.

“The story in the New York Times regarding Jim Webb being considered as the next Secretary of Defense is FAKE NEWS,” Trump tweeted. “I’m sure he is a fine man, but I don’t know Jim, and never met him. Patrick Shanahan, who is Acting Secretary of Defense, is doing a great job!”

Shanahan is a former Boeing executive who has little experience of diplomacy or management of the military.

Elsewhere, David Bernhardt is acting secretary of the interior department, Matthew Whitaker is acting attorney general, Andrew Wheeler is acting head of the Environmental Protection Agency and Mick Mulvaney is the acting White House chief of staff. Jonathan Cohen is acting ambassador to the United Nations.

According to the Brookings Institution, a Washington thinktank, in the first two years of Trump’s presidency there has been a 65% turnover rate among senior-level advisers.