PHILADELPHIA — On a chaotic sixth day of Donald Trump’s presidency, the White House on Thursday appeared to back a 20 percent tax on all imports from Mexico to the United States to pay for a proposed border wall, only to insist a few hours later that it was not endorsing the plan.

The mixed messages from White House press secretary Sean Spicer came hours after Mexico’s president pulled out of next week’s planned visit to Washington, D.C., part of a day that also saw a wave of resignations by senior State Department officials, a presidential visit to Philadelphia that was greeted by huge street demonstrations, and White House chief strategist Stephen Bannon telling The New York Times that the media should “keep its mouth shut.’’

Separately, the Times reported that the Trump White House is drafting a presidential directive that calls on Defense Secretary James N. Mattis to devise plans to more aggressively strike the Islamic State, which could include US artillery on the ground in Syria and Army attack helicopters to support an assault on the group’s capital, Raqqa.

Trump, who is to make his first visit to the Pentagon as commander in chief on Friday, will demand that the new options be presented to him within 30 days, the officials said. During the presidential campaign, Trump repeatedly said that he had a secret plan to defeat the Islamic State, but he also said that he would give his commanders a month to come up with other options.

The White House is also expected to press for a review of the U.S. nuclear posture — one that retains all three legs of the nuclear arsenal, with weapons aboard bombers and submarines and in underground missile silos — as well as a review of how to achieve the president’s goal of fielding a “state of the art’’ anti-missile system.

Trump traveled to Philadelphia Thursday to attend a retreat for GOP members of the House and Senate where he called on fellow Republicans to help him enact ‘‘great and lasting change.’’

The president was greeted by cheers as he took the stage in a hotel ballroom, telling lawmakers, ‘‘This Congress is going to be the busiest Congress in decades — maybe ever.’’

He addressed lawmakers shortly after Mexican President Enrique Peña Nieto canceled a trip to Washington next week for his first meeting with the new president due to their disagreement over which of their countries would pay to build Trump’s promised wall on the border between them.

The wall is part of Trump’s plan to halt illegal immigration to the United States, and he has long insisted that Mexico will pay. Peña Nieto insists his country will not.

On the flight back to Washington, Spicer told reporters traveling with the president that Trump will seek a 20 percent tax on Mexican imports to pay for the wall, the most specific proposal Trump has ever floated for how to cover a project estimated to cost between $12 billion and $15 billion. Congressional approval would be needed for such a step.

‘‘By doing that, we can do $10 billion a year and easily pay for the wall just through that mechanism alone,’’ Spicer said. ‘‘This is something that we've been in close contact with both houses in moving forward and creating a plan.’’

But the announcement sparked immediate confusion across Washington, and the White House tried to backtrack. At the White House, Spicer and other officials tried to take back his earlier comments.

During a hastily arranged briefing in the West Wing, chief of staff Reince Priebus said a 20 percent import tax was one idea in ‘‘a buffet of options’’ to pay for the border wall.

Business groups and lawmakers immediately threw cold water on the tax, saying it would ultimately be passed on to US consumers.

“Simply put, any policy proposal which drives up costs of Corona, tequila, or margaritas is a big-time bad idea. Mucho Sad,’’ Republican Senator Lindsey Graham of South Carolina tweeted.

In Trump’s remarks to lawmakers, he cast the cancellation of his engagement with Pena Nieto as a mutual decision.

He said that ‘‘unless Mexico is going to treat the United States fairly, with respect, such a meeting would be fruitless, and I want to go a different route. We have no choice.’’

Both Peña Nieto and Spicer said on Thursday that their countries were interested in maintaining positive relations. ‘‘We will keep the lines of communication open,’’ Spicer told reporters in Washington, adding that the White House would ‘‘look for a date to schedule something in the future.’’

Mexicans view a wall across the 2,000-mile border as a symbolic affront, part of a package of Trump policies that could cause the country serious economic pain.

They include a crackdown on illegal immigrants, who send billions of dollars home, and renegotiation of the North American Free Trade Agreement, or NAFTA.

The treaty has allowed trade between the neighbors to mushroom. Every day, goods valued at $1.4 billion cross the US-Mexico border, and millions of jobs are linked to trade on both sides.

Mexico is the world’s second-largest customer for American-made products, and 80 percent of Mexican exports — automobiles, flat-screen TVs, avocados — are sold to the United States.

Major harm to Mexico’s economy could spur more people to somehow cross the border to the United States — undercutting Trump’s major goal of stopping illegal immigration.

Prime Minister Theresa May of Britain also spoke at the Republican gathering, embracing Trump as a friend and ally, but cautioning him not to turn his back on global institutions and long-established political values.

On her first visit to the United States as prime minister, May called the start of Trump’s term ‘‘a new era of American renewal’’ — but firmly rejected the president’s suggestion that torture of suspected terrorists might be acceptable, and rebuffed some of his foreign-policy views.

May flew to Philadelphia a day before she will hold talks with Trump at the White House and become the first foreign leader to meet the president since his inauguration.

Thousands of anti-Trump protesters marched in the area around the hotel where the retreat was being held, The Philadelphia Inquirer reported.

The State Department said Thursday that several senior management officials as well as a top arms control diplomat would be leaving the agency.

All had submitted their resignations prior to Trump’s Jan. 20 inauguration as is required of officials holding jobs appointed by the president.

They were not required to leave the foreign service but chose to retire or resign for personal reasons, the department said.

While none of the officials has linked their departure explicitly to Trump, many diplomats have privately expressed concern about serving in his administration given the unorthodox positions he’s taken on many foreign policy issues.

Turnover among senior leadership during presidential transitions is not unusual, although the career diplomats who are leaving the foreign service entirely had served under both Republican and Democratic presidents.

In an interview published by The New York Times on Thursday, Bannon argued that news organizations “should be embarrassed and humiliated and keep its mouth shut and just listen for a while.’’

“I want you to quote this,’’ Bannon added. “The media here is the opposition party. They don’t understand this country. They still do not understand why Donald Trump is the president of the United States.’’

The conversation was initiated by Bannon to offer praise for Spicer, who has been criticized this week for making false claims at the White House podium.

Asked whether he was concerned that Spicer had lost credibility with the press, Bannon chortled. “Are you kidding me?’’ he said. “We think that’s a badge of honor. ‘Questioning his integrity’ — are you kidding me? The media has zero integrity, zero intelligence, and no hard work.’’

Material from The New York Times, The Washington Post, and the Associated Press was included in this report.