Trump, who spent part of the day at Camp David for staff meetings, showed no signs of budging on his demand of more than $5 billion for a wall along the U.S.-Mexico border. White House officials affirmed that request in a letter to Capitol Hill after a meeting with senior congressional aides led by Vice President Mike Pence at the White House complex yielded little progress.

The letter from Office of Management and Budget Acting Director Russell Vought also formalized Trump's declaration that the wall would be built from steel, rather than concrete, asking for funding for a "steel barrier on the Southwest border."

The White House said the letter, as well as details provided during the meeting, sought to answer Democrats' questions about the funding request. Democrats, though, said the administration failed in both the meeting and the letter to provide a full budget of how it would spend the billions requested on the wall, money the president wants from Congress.

The letter includes a request for $800 million for "urgent humanitarian needs," a reflection of the growing anxiety over migrants traveling to the border — which the White House said Democrats raised in the meetings. And it repeats some existing funding requests for detention beds and security officers, which have already been panned by Congress and would likely find resistance among House Democrats.

Still, the request makes clear a wall is a top priority. Vought writes that a "physical barrier — wall — creates an enduring capability that helps field personnel stop, slow down and/or contain illegal entries."

Trump sought to frame a steel barrier as progress as he returned from the presidential retreat in Maryland, saying Democrats "don't like concrete, so we'll give them steel." The president has already suggested his definition of the wall is flexible, but Democrats have made clear they see a wall as immoral and ineffective and prefer other types of border security funded at already agreed upon levels.

With the partial shutdown in its third week, House Speaker Nancy Pelosi said she intends to begin passing individual bills to reopen agencies in the coming days, starting with the Treasury Department to ensure people receive their tax refunds. That effort is designed to squeeze Senate Republicans, some of whom are growing increasingly anxious about the extended shutdown.

The seemingly intractable budget showdown marks the first clash for Trump and Democrats, who now control the House. It pits Trump's unpredictable negotiating stylings against a largely united Democratic front, as many Republicans watch nervously from the sidelines and hundreds of thousands of federal workers go without pay.

Although Trump tweeted that the Sunday session had been "productive," two Democrats familiar with the meeting gave a different take, saying the White House had not provided the budget details they had requested and again declined to re-open government. One of the officials — neither was authorized to speak publicly — said no additional meetings were scheduled.

Trump said earlier in the day that he was hoping for "some very serious talks come Monday, Tuesday, Wednesday." While insisting he wanted to make a deal, he also declared he would not give an inch in his fight for funding for a border barrier, saying: "There's not going to be any bend right here."

Among the Republicans expressing concerns was Sen. Susan Collins, R-Maine, who said Senate Majority Leader Mitch McConnell should take up bills from the Democratic-led House.

"Let's get those reopened while the negotiations continue," Collins said on NBC's "Meet the Press." Democrats criticized McConnell for waiting on Trump's support, but Collins said she was sympathetic to McConnell's opposition to moving legislation without agreement from the president.

Several Republicans pushed the Interior Department to find money to restaff national parks amid growing concerns over upkeep and public safety. And Senate Minority Leader Chuck Schumer, D-N.Y., suggested Sunday that pressure would only mount amid the shutdown, which he said is disrupting Transportation Security Administration operations, home loans and farmers in his state.