MANAMA (Reuters) - Defence Secretary Jim Mattis will stress the U.S. commitment to Middle East security at the weekend, officials said, amid Western concern that any loosening of U.S. ties to Riyadh after Jamal Khashoggi’s murder could allow Russia to fill the vacuum.

FILE PHOTO: U.S. Defense Secretary James Mattis (R) waits to welcome Saudi Arabia's Deputy Crown Prince and Minister of Defense Mohammed bin Salman at the Pentagon in Washington, U.S., March 16, 2017. REUTERS/Yuri Gripas/File Photo

Mattis will give a speech at the annual Manama Dialogue security conference in Bahrain on Saturday on the United States’ policy in the Middle East which will be closely watched in capitals around the region.

Katie Wheelbarger, the principal deputy assistant secretary of defense for international security affairs, said that Mattis’ speech would emphasize that the United States was committed to help allies “re-instill stability in a chaotic region.”

“So continue to look to the United states as your security partner of choice because you can rely on us and depend on us to be there long term,” Wheelbarger told reporters traveling with Mattis.

There is increasing concern that if Washington abandons the region, countries like Russia will fill the vacuum.

Nowhere will the speech be watched more closely then in Riyadh, after Saudi Arabia’s public prosecutor said the murder of journalist Jamal Khashoggi in the kingdom’s Istanbul consulate this month was premeditated.

The death of Khashoggi, a Washington Post columnist and critic of de facto Saudi ruler Crown Prince Mohammed bin Salman, has sparked global outrage and mushroomed into a crisis for the world’s top oil exporter and strategic ally of the West.

Riyadh is expected to be watching how far, if at all, Mattis goes in censuring the kingdom in his speech.

After Saudi King Salman spoke to Russian President Vladimir Putin on Thursday about the killing, the Kremlin said Russia had no reason to doubt the statements of the king and crown prince that the royal family was not involved.

Dozens of Western officials, bankers, and executives shunned a big investment conference in Riyadh this week.

Russia sent the head of its Russian sovereign wealth fund RDIF, who said Saudi Arabia’s Public Investment Fund would become a new partner in a joint Russia-China Investment Fund.

Saudi Arabia is the lynchpin of a U.S.-backed regional bloc against Iran but the crisis has strained Riyadh’s relations with the West.

Experts believe that Washington will have to take some measures to make clear to Saudi Arabia that Khashoggi’s killing is unacceptable.

“I think the key here is that if the administration wants to shift the focus back onto Iran the only way to do that is by taking some steps to respond to what the Saudis have done,” Dennis Ross, who served as top Middle East adviser to President Barack Obama, said.

Ross said the increased global pressure on Riyadh may also present an opportunity for Washington to push the kingdom to end a 16-month-old dispute between Qatar and four Arab states that analysts say has weakened regional coordination against Iran.

Saudi Arabia, the Emirates, Bahrain and Egypt cut off travel and trade ties with Qatar in June 2017, accusing it of backing their arch-rival, Iran, and supporting terrorism. Qatar denies the charges and says the boycott impinges on its sovereignty.

The United States has tried, without success, to mediate in the dispute. It is an ally of the six-nation Gulf Cooperation Council, and Qatar is home to a major U.S. air base.

“This might not be a bad time for the administration to try to go back in and mediate that dispute, this might be a point where the Saudis will want to be more responsive to us,” Ross said.