President Trump flew out of Turkey on Wednesday night on the old Air Force One instead of his new Qatari-donated Boeing 747-8 as a security precaution related to the resumption of hostilities with Iran, according to people briefed on the plans, who said the change came at the urging of the Secret Service.

The swap deepens questions about whether the new plane, which the president had pressed to be ready as soon as possible, was retrofitted with sufficient security measures over the last year. Lawmakers and some officials have raised concerns about whether the expedited timeline allowed for the addition of an advanced missile defense system and other modifications used to protect the president.

In a statement, Steven Cheung, the White House communications director, said that “the new Air Force One is a state-of-the-art aircraft that has been fitted with high-level security protocols that ensure the safety of the president and his staff.”

“As the president has said recently, there are many enemies of America who have their sights on him, and we use every tool at our disposal — including distraction and misdirection — to address those threats,” he added.

But people briefed on the new plane’s capabilities, who spoke on the condition of anonymity to discuss sensitive security issues, said the new plane does not have all the features of the older plane. The switch in the president’s aircraft when he departed Turkey was a precautionary measure made at the advice of the Secret Service and not because of a specific threat, they said.

Mr. Trump, who has marveled at the luxury touches of his new jet, flew on it on Monday night to go to Turkey for a NATO summit. After his arrival, the conflict with Iran reignited, and the United States launched a series of strikes against that country while Mr. Trump and NATO leaders were about 1,000 miles away in Ankara.

The president on Wednesday denied that the change in his aircraft was made because of security concerns. Instead, he asserted that the swap was so the new jet could leave early and make stops at U.S. military bases to show it off to the troops because the aircraft is “magnificent.”

But when pressed by reporters in Ankara about the reason for the change, Mr. Trump also repeatedly noted that he was Iran’s No. 1 target, and referred at one point to having seen or been briefed on a list of Tehran’s targets in recent days.

Earlier Wednesday, Mr. Trump wrote in a social media post that he would fly aboard the older aircraft “for old time’s sake” out of Ankara so that the newer plane could be shown off at Mildenhall Air Force Base in England to give U.S. troops a “chance to tour the Aircraft.”

The Secret Service declined to comment, referring to the president’s post as the explanation for the change.

When Mr. Trump departed Ankara, he boarded the old aircraft unusually quickly, before the journalists traveling with him could watch or photograph him ascending the steps, as they typically do. Passengers on board were also instructed to pull their window shades down before takeoff.

The plane landed at Mildenhall late Wednesday night, and then the president switched to the new jet to return to Washington.

After Mr. Trump departed, he told reporters that they had probably been instructed to close their blinds when leaving Ankara because they were “on a dangerous plane” due to the threat from Iran.

The older plane has been widely reported to be equipped with a system designed to blind an incoming antiaircraft missile, along with “chaff” that could be deployed to mislead a missile and force it off course.

It is unclear how many, if any, of those capabilities have been installed on the newer plane that was donated from Qatar, which Mr. Trump has been eager to get into service.

Industry and Pentagon officials have said such an extensive upgrade could cost as much as $1 billion and take up to two years to complete. But in testimony before Congress, Troy E. Meink, the Air Force secretary, estimated that the modifications would run “probably less than $400 million.”

The Air Force started upgrading the 747 jetliner in the United States last summer.

Air Force officials said at the time that they were modifying the jet for “executive airlift” support, on orders of Defense Secretary Pete Hegseth, but that all other details about the upgrade were classified.

The plan immediately drew criticism from some members of Congress. The lawmakers expressed concern that Mr. Trump would pressure the Air Force to do the work so fast that sufficient security measures would not be built into the plane, including missile defense systems or even systems to protect it from the electromagnetic effects of a nuclear blast.

Much of the retrofitting and refurbishing that the Air Force described took place at a Texas facility known for secret technology projects.

Andrew P. Hunter, the former Air Force assistant secretary who was in charge of the Air Force One program during the Biden administration, said that a true retrofit of a 747 jet to prepare it to become Air Force One would require more than a year of work.

That is because the base plane, even if it has a luxurious interior, needs significant modifications to its physical structure to accommodate special security upgrades — work that would have taken longer than the Air Force had to retrofit the donated Qatari jet.

Mr. Hunter would not name the more complicated security upgrades individually because they are considered classified. But other government officials have told The New York Times that this more complex work includes advanced missile defense systems and hardening of the plane’s wiring to protect it from an electromagnetic pulse, in case there is a nuclear strike. It is unclear whether such work was done to the Qatari jet.

“In that time they had, they would be able to accommodate communications upgrades,” Mr. Hunter said, referring to vital security equipment that allows a president to be in contact at all times. “But not anything that would require significant structural work.”

Mr. Hunter added: “To do a full Air Force One equivalent upgrade does require structural modifications.”

The Times asked the Air Force earlier this year whether these kinds of measures had been included in the updates to the Qatari jet. The Air Force declined to respond to the questions.

Doug Mills and David E. Sanger contributed reporting.