President Donald Trump has announced the US will lift crippling sanctions on Syria to give the war-torn nation “a chance at greatness” on the first foreign trip of his presidency.

Trump made the pledge during his visit to Saudi Arabia, where he also announced a record $US142 billion defence sales deal with that kingdom.

Trump picked Saudi Arabia for the first stop on his tour of three Gulf countries because it has promised to make big investments in the US.

Saudi Arabia, Qatar and the United Arab Emirates are all places where the Trump Organisation, run by his two elder sons, is developing major real estate projects.

They include a high-rise tower in Jeddah, a luxury hotel in Dubai and a golf course and villa complex in Qatar.

On day one of the four-day visit, Trump signed the arms deal with Saudi Arabia and other investments that could be worth $US1 trillion.

Trump also announced the US would lift all sanctions on Syria following the fall of the Assad regime, a decision he made after speaking to Saudi Crown Prince Mohammed bin Salman and Turkish President Recep Tayyip Erdogan.

“Syria, they’ve had their share of travesty, war, killing in many years. That’s why my administration has already taken the first steps toward restoring normal relations between the United States and Syria for the first time in more than a decade,” he said.

“The sanctions were brutal and crippling and served as an important — really an important function — nevertheless, at the time.

“But now it’s their time to shine. So I say, ‘Good luck, Syria.’ Show us something very special.”

Saudi Arabia’s de facto ruler, Crown Prince Mohammed bin Salman, warmly greeted Trump on Tuesday as he stepped off Air Force One.

The two leaders then retreated to a grand hall at the Riyadh airport for a coffee ceremony. The crown prince feted Trump with a formal dinner.

Trump was accompanied on the trip by Elon Musk and representatives from Boeing and Google.

On Wednesday, Trump will join a gathering of members of the Gulf Co-operation Council — made up of Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the United Arab Emirates — before leaving Riyadh.

Riyadh was also the first overseas stop of his first term.

Trump defends gifted jet

Meanwhile, Trump appears ready to accept a luxury Boeing 747-8 jumbo jet as a gift from the ruling family of Qatar during his trip to the Middle East, and officials say it could be converted into a potential presidential aircraft.

The Qatari government acknowledged discussions between the two countries about “the possible transfer” of a plane to be used temporarily as Trump’s Air Force One, but denied the jet “is being gifted” and said no final decision had been made.

In a post on his social media site Truth Social, Trump appeared to confirm the proposal.

“So the fact that the Defense Department is getting a GIFT, FREE OF CHARGE, of a 747 aircraft to replace the 40 year old Air Force One, temporarily, in a very public and transparent transaction, so bothers the Crooked Democrats that they insist we pay, TOP DOLLAR, for the plane,” he wrote.

White House press secretary Karoline Leavitt said in a statement: “Any gift given by a foreign government is always accepted in full compliance with all applicable laws. President Trump’s administration is committed to full transparency.”

ABC News reported that Trump will use the plane as a new version of Air Force One until shortly before he leaves office in January 2029, when ownership will be transferred to the foundation overseeing his yet-to-be-built presidential library.

Ali Al-Ansari, Qatar’s media attache, in a statement said, “Reports that a jet is being gifted by Qatar to the United States government during the upcoming visit of President Trump are inaccurate.”

“The possible transfer of an aircraft for temporary use as Air Force One is currently under consideration between Qatar’s Ministry of Defence and the US Department of Defence,” the statement said.

“But the matter remains under review by the respective legal departments, and no decision has been made.”

Meanwhile, administration officials, anticipating questions about the president accepting such a large gift from a foreign government, have prepared an analysis arguing that doing so would be legal, according to ABC.

The Constitution’s Emoluments Clause bars anyone holding government office from accepting any present, emolument, office or title from any “king, prince, or foreign state”, without congressional consent.

One expert on government ethics, Kathleen Clark of the Washington University School of Law in St Louis, accused Trump of being “committed to exploiting the federal government’s power, not on behalf of policy goals, but for amassing personal wealth”.

“This is outrageous,” Clark said.

“Trump believes he will get away this.”

Senate Minority Leader Chuck Schumer poked fun at Trump’s “America first” political slogan.

“Nothing says ‘America First’ like Air Force One, brought to you by Qatar,” the New York Democrat said in a statement. “It’s not just bribery, it’s premium foreign influence with extra leg room.”

Trump intends to convert the Qatari aircraft into a plane he can fly on as president, with the Air Force planning to add secure communications and other classified elements to it.

But it will still have more limited capabilities than the existing planes that were built to serve as Air Force One, as well as two other aircraft currently under construction, according to a former US official.

ABC said the new plane is similar to a 13-year-old Boeing aircraft Trump toured in February, while it was parked at Palm Beach International Airport and he was spending the weekend at his Mar-a-Lago club.

-with AAP/Reuters