President Barack Obama on Tuesday left the United States for Saudi Arabia, the first stop on a six-day, three-nation trip that will also take him to Britain and Germany.
In Saudi Arabia, Obama will consult Washington's Gulf allies on the crises in Yemen and Syria, but he is unlikely to receive a warm reception.
The US president will meet King Salman on Wednesday, before a Gulf Cooperation Council summit of leaders from Saudi Arabia, Bahrain, Kuwait, the UAE, Qatar and Oman on Thursday.
Saudi Arabia has been frustrated with Obama on a number of issues -- most notably his deal with Shia power Iran to end its economic and diplomatic isolation in return for curbs of its nuclear program.
Obama will then travel to Britain, where he will meet Prime Minister David Cameron two months ahead of the country's referendum on European Union membership, and have lunch with Queen Elizabeth II at Windsor Castle.
First Lady Michelle Obama will join her husband in London, the White House said.
If Britain were to leave the EU -- in a so-called Brexit -- it could have deep ramifications for Washington's "special relationship" with Britain, and for the stability of the European Union itself.
The final stop of the trip will be Germany, where he will meet Chancellor Angela Merkel and speak at the opening of the Hanover industrial technology fair.