MANAMA (Reuters) - U.S. Defense Secretary Jim Mattis said on Saturday that the killing of Saudi journalist Jamal Khashoggi undermined Middle Eastern stability and that Washington would take additional measures against those responsible.

Washington Post columnist Khashoggi’s murder has escalated into a crisis for the world’s top oil exporter. Saudi Arabia’s allies have reacted with outrage toward a country that is the lynchpin of a U.S.-backed regional bloc against growing Iranian influence in the Middle East.

But Mattis also said U.S. respect for the Saudi people was undiminished, while Saudi Foreign Minister Adel al-Jubeir said those behind the killing would be prosecuted in the kingdom and that the investigation would take time.

U.S. President Donald Trump has said he wants to get to the bottom of the case, while also highlighting Riyadh’s role as an ally against Tehran and Islamist militants, as well as a major purchaser of U.S. arms.

“With our collective interests in peace and unwavering respect for human rights in mind, the murder of Jamal Khashoggi in a diplomatic facility must concern us all greatly,” Mattis told a conference in Bahrain.

“Failure of any one nation to adhere to international norms and the rule of law undermines regional stability at a time when it is needed most,” Mattis said. He did not mention de facto Saudi ruler Crown Prince Mohammed bin Salman by name at any point.

Saudi Arabia’s public prosecutor said Khashoggi’s killing was premeditated, contradicting a previous official statement that it happened accidentally during a tussle in the Saudi consulate in Istanbul.

Saudi officials have also said he was accidentally killed in a botched security operation to return him to the kingdom.

In his remarks at the Manama Dialogue security conference, Mattis went through a list of what he described as disruptive Iranian behavior - a message most Gulf allies will view positively since they share similar concerns about Iran’s increasing influence in Syria and Iraq.

While these were some of the sharpest comments Mattis has made on the Khashoggi killing, he also said the two countries still needed to collaborate on stability in the region.

“It’s hard to imagine that this administration in particular is going to change fundamentally how it views the role of the Saudis in terms of counterterrorism, in terms of counter-Iran,” said Dennis Ross, who served as top Middle East adviser to President Barack Obama in his first term.

Foreign Minister Jubeir, speaking at the same conference, said Riyadh’s relations with Washington were “ironclad” amid what he called “hysteria in the media” over Khashoggi’s killing.

In response to the killing, U.S. Secretary of State Mike Pompeo this week announced moves against 21 Saudis to either revoke their visas or make them ineligible for U.S. visas after the Khashoggi killing.

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“Our Secretary of State ...will be taking additional measures as the situation is clarified,” Mattis said.

ALLIANCES

Mattis said the presence in the Middle East of Russia - a major ally of Syrian President Bashar al-Assad - could not be a replacement for the United States, whose “long-standing, enduring, and transparent” commitment to the region he reiterated.

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He said that it was important 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 archrival, Iran, and supporting terrorism. Qatar denies the charges.

“The solving of internal debates among our GCC partners is vital for realizing this vision. Without it, we weaken our security,” he said, referring to the Gulf Cooperation Council nations.

Mattis said he continued to support partners in the region who were defending themselves against Houthi attacks in Yemen but also called for an end to fighting there.

A Saudi-led coalition that intervened in Yemen’s war in 2015 has conducted frequent air strikes targeting the Iran-aligned Houthi group and has often hit civilians, although it denies doing so intentionally.