US Secretary of State Mike Pompeo was accused Monday of using US foreign policy to support President Donald Trump's reelection bid after embarking on a trip to the Middle East, where he plans to remotely deliver an address to the Republican National Convention.

Pompeo suddenly flew out for a six-day trip to Israel, Bahrain, the United Arab Emirates and Sudan on Sunday, with the precise itinerary still uncertain, the reason for the urgency unclear and only one reporter in tow, when normally there are several.

"The US commitment to peace, security, and stability in Israel, Sudan, and among Gulf countries has never been stronger than under President Trump's leadership," the State Department said, announcing the trip.

Pompeo was meeting Israeli Prime Minister Benjamin Netanyahu and other officials on the first stop in Jerusalem on Monday.

Officially, the mission was to herald the "historic" accord between Israel and the United Arab Emirates which Trump has called a success of his administration's Middle East peace efforts.

The administration says it is optimistic other Arab countries will follow suit and recognize Israel, and Pompeo's trip appears aimed to step up pressure on Bahrain and Sudan towards that end.

But with polls showing Trump well behind Democratic rival Joe Biden in the race, critics say the Republican president is seeking to use Middle East diplomacy to his domestic political advantage.

He is particularly aiming at building more support among Jewish and evangelical Christian voters with his groundbreaking recognition of the holy city of Jerusalem as Israel's capital in 2017.

Foreign policy 'stain'

But Pompeo's trip, which coincides with the opening Monday of the Republican convention to officially designate Trump the party candidate in the November 3 election, has raised questions among diplomacy experts and especially Democrats.

While on his fifth visit to Israel, Pompeo will reportedly record an address to the convention to be shown Tuesday.

"Secretaries of state have long taken pains to avoid mixing diplomacy with domestic politics," said Democratic Senator Bob Menendez.

"Doing so puts a stain on our nation's foreign policy and our electoral process," he said on Twitter.

Halie Soifer, director of the Jewish Democratic Council of America, called it "unprecedented and highly unethical" for Pompeo to address the Republican fest while on a diplomatic mission.

With Pompeo's trip, she said, "Trump is once again using Israel to score political points."

She pointed to Trump's statement last week that he had controversially moved the US embassy in Israel from Tel Aviv to Jerusalem only to impress conservative American Christians, a core group of his support base.

"We moved the capital of Israel to Jerusalem. That's for the evangelicals," Trump said.

"The evangelicals are more excited about that than Jewish people," he added.

Pompeo's staff had no comment on the criticisms, some of which cited a law, the Hatch Act, which bans officials from engaging in electoral politics while undertaking official duties.

The State Department said Pompeo will address the convention in his personal and not official capacity.

"No State Department resources will be used. Staff are not involved in preparing the remarks or in the arrangements for Secretary Pompeo's appearance," an official told AFP in a statement.

The official added that, even as it happens during an official trip, the State Department would not "bear any costs" associated with the statement.

Some critics suspected the entire trip was arranged as a show for the convention, which will see Trump and top Republicans appearing on television for four nights straight to extol his record.

Normally a secretary of state would appear in person to deliver a powerful message for an incumbent president.