(CNN) -- Four US officials said President Donald Trump's decision to leave Turkey on the old presidential plane, while sending his new plane, a gift from Qatar, ahead to England, was driven -- at least in part -- by security concerns linked to escalating conflict with Iran.

Two officials explained that the security team felt more comfortable with the president on the older plane -- which was built from scratch with the commander in chief's safety in mind -- compared to the plane that underwent modifications and modern fittings after Qatar donated it.

While sources said there is no new, specific threat to Trump's life, the president alluded to the possibility of an assassination attempt by Iran during the NATO summit, following new US strikes targeting the country that shares a border with Turkey.

Trump said: 'I'm number one on the hit list.'

The New York Times had previously reported on these security concerns, and CNN reached out to the White House for comment.

It was not immediately clear the specific reason why the older plane was considered a safer option than the newer one. Two informed sources said the new plane does not have the same capabilities needed to ensure its safety in international airspace, while a third source stated it was equipped with many of the same features as the older plane.

One source said: 'Threat assessments are constantly changing.'

Before Trump left for the NATO summit in Turkey, officials sent the Qatari plane.

Before Trump left for the NATO summit in Turkey, officials sent the Qatari-donated plane to Mildenhall Air Base in England. Trump said in a social media post that the change in planes was simply to give US service members stationed at the base 'a chance to tour the plane.'

He wrote: 'Everyone is very excited, and we think they should be the first to see it.'

Afterward, Trump swapped the two planes at a secure US air base in the United Kingdom, but he gave conflicting statements about why the plans changed.

He said: 'There were no security concerns, except that we sent the plane a little early... We sent it a bit early so they could see it.'

But when asked why accompanying journalists on the plane were told to close the window shades during takeoff from Ankara, Trump acknowledged that security concerns related to Iran may have been a factor. He said the presidential plane, Air Force One, could be in a 'dangerous situation because of the scum we have to deal with.'

He added: 'They are sick people, so I could imagine something like that happening,' noting that he was not aware of instructions for the media delegation members to keep the window shades closed.

The new plane -- a gift from Qatar valued at about $400 million -- raised legal, ethical, and national security questions, despite the Air Force's assurance that it is 'safe, secure, and equipped with the latest technology needed to meet presidential mission requirements.'

One official, who declined to comment on the highly classified systems typically on board the presidential plane, indicated that the 747 donated by Qatar was largely viewed by the military and the Secret Service as having been rushed into service.

In contrast, the official noted that two new planes -- acquired under a deal Trump renegotiated during his first term -- have faced significant delays and are not expected to be ready before at least 2028. This is partly due to the suite of secure communications systems and countermeasure defenses that need to be installed, the military's comprehensive design requirements, and the time needed to train pilots on these unique aircraft.

Another source said about the Qatari plane that it was always considered a 'transitional plane' for the president until the other aircraft are fully equipped. Compared to the two new Boeing planes that have been undergoing modifications for years, the modification of the Qatari plane took only about a year and a half, raising concerns among some security officials at the time.

The source indicated that -- during private discussions long before this trip -- senior White House officials discussed the possibility of using the older plane instead of the Qatari one for international flights that pose high risks, if necessary.

The source said: 'The new plane is completely safe for the president to travel on,' but added that additional precautions are required when traveling to high-risk areas internationally.

The older plane that flew Trump from Turkey -- a heavily modified Boeing 747-200 -- has highly specialized capabilities, including in-flight refueling.