Civil liberties monitors point to freedom of expression as a right increasingly constrained in Gulf Arab states including summit host Saudi Arabia, which is planning to buy tens of billions of dollars' worth of U.S. arms.

Gulf Arab states began stepping up the muffling of political discussion in the dying months of former president Barack Obama's term and have continued this under Trump, they say.

"Given Trump's tenuous relationship with freedom of the press and free expression in general, we have no expectation that Trump would raise these issues during his visit," said Adam Coogle, Middle East researcher at Human Rights Watch.

In Washington, a senior Trump administration official said human rights would not take center stage in Riyadh, where Arab leaders are expected to discuss combating Islamist militancy and what they see as the growing influence of adversary Iran.

The official said Trump preferred to keep such conversations private, much as he did with Egyptian President Abdel Fattah al-Sisi recently when he obtained the release of an Egyptian-American humanitarian worker.

Trump's visit is likely to contrast with one Obama paid to Egypt in 2009 when he made an appeal to the Muslim world promoting self-determination, democracy and individual rights.

The Saudis "don't want any more talk about human rights, democracy, political reform or gender equality. They had enough of that from Barack Obama and Hillary Clinton," said Bruce Riedel of the Brookings Institution think tank in Washington.

"They're pretty confident they're not going to hear it from Donald Trump."

While experts are not surprised, since the Gulf states' monarchies abhor discord and dislike free-wheeling political debate as practised in the West, they are nevertheless dismayed.

The output of several columnists, economists and clerics in regional powerhouse Saudi Arabia and some of its smaller neighbors has either dried up or grown circumspect since the second half of 2016 in what critics see as an unacknowledged state drive to stifle public criticism, rights monitors say.

Among those who have fallen silent are critics, both liberal

and conservative, of the kingdom's ambitious plan to diversify the economy and open up the country culturally under a plan known as Vision 2030.

Until late last year Saudi writer Jamal Khashoggi commented about issues including Trump's rise to power on social media and a column in the pan-Arab al-Hayat daily. He also spoke in public appearances at think tanks.