Saudi Arabia and key ally Bahrain said on Saturday that Gulf states are playing a critical role in maintaining stability in the Middle East by combating Iran's "vision of darkness", as Riyadh faces its worst political crisis in decades.

Saudi Arabia is the lynchpin of a U.S.-backed regional bloc against growing Iranian influence in the Middle East but the murder of journalist Jamal Khashoggi at its consulate in Istanbul on Oct. 2. has prompted a global outcry and strained Riyadh's ties with the West.

"We are now dealing with two visions in the Middle East. One is a (Saudi) vision of light ... One is a (Iranian) vision of darkness which seeks to spread sectarianism throughout the region," Foreign Minister Adel al-Jubeir told a security summit in Bahrain.

"History tells us that light always wins out against the dark ... The question is how do we defeat them."

U.S. Defense Secretary Jim Mattis said on Saturday that the killing of Khashoggi, a prominent critic of Saudi policy, undermined regional stability and that Washington would take additional measures against those responsible after announcing visa bans on 21 suspects in the case.

"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 told a separate session of the annual Manama Dialogue security conference.

The long-planned event has been the focus of greater scrutiny this year after the killing of Khashoggi, with Jubeir questioned about how the case would affect Saudi Arabia's credibility on foreign policy and security matters.

He reiterated that Riyadh would bring to justice those responsible for Khashoggi's death, saying they would be prosecuted in the kingdom, a day after Turkey demanded the extradition of 18 Saudis suspected of involvement in his murder.

Jubeir said ties between Saudi Arabia, the world's top oil exporter, and the United States are "ironclad" and praised what he described as the "rational, realistic" foreign policy of the current U.S. administration.

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

Sunni Muslim Saudi Arabia and Shi'ite Muslim Iran, which was not represented in the Manama conference agenda, have long been locked in a proxy war, competing for regional supremacy from Iraq to Syria and Lebanon to Yemen.

Trump, who in May withdraw the United States from a 2015 international nuclear accord with Tehran, has strongly backed Saudi Arabia in its efforts to counter Iran's influence. The next wave of U.S. sanctions against Iran are due to come into effect on Nov 4.

Iran, for its part, accuses Saudi Arabia and the United Arab Emirates of funding gunmen who attacked a military parade in Iran last month, killing 25 people, 12 of them members of the elite Revolutionary Guards. Saudi Arabia and the UAE have denied any involvement.

The Guards accused them of creating "plots and tensions".

SECURITY ALLIANCE

Bahrain's foreign minister, Shiekh Khalid bin Ahmed Al Khalifa, told the conference the Gulf bloc would remain a "pillar" of regional security and that a proposed security alliance grouping the United States, Gulf states, Jordan and Egypt would be activated next year.

The Middle East Strategic Alliance (MESA) is meant to serve as a bulwark against Iran and extremism, Washington says. But there has been uncertainty about how it can get off the ground given a protracted dispute between Qatar and four Arab states led by Saudi Arabia who launched a boycott of Doha in 2017.

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 and says the boycott impinges on its sovereignty.

Mattis said that it was important to end the 16-month-old Arab dispute with Qatar that analysts say has weakened regional coordination against Iran.

Jubeir said recent discussions in Saudi Arabia about a framework for MESA included Qatari officials and that the proposed alliance would not be affected by the diplomatic row.

"It (MESA) is an alliance for security and prosperity for the region and will be open to those who accept its principles," Sheikh Khalid said, adding that the alliance would also cooperate on economic issues.

Gulf Arab foreign ministers also touched on Middle East peace initiatives, with Sheikh Khalid lauding efforts by Oman, which hosted Israeli Prime Minister Benjamin Netanyahu on Friday in a rare visit.

Oman's foreign minister, Yousuf bin Alawi bin Abdullah, told the forum that Muscat was offering ideas to help Israel and the Palestinians to come together but was not acting as mediator.