Russia is no replacement for the United States in the Middle East following Moscow's military intervention in Syria, US Defence Secretary Jim Mattis told a regional security conference on Saturday.
"Russia's presence in the region cannot replace the longstanding, enduring, and transparent US commitment to the Middle East," Mattis told a meeting in the Bahraini capital Manama.
He was speaking as Russian President Vladimir Putin was due to hold talks in Istanbul with the leaders of France, Germany and Turkey on Saturday seeking to find a lasting political solution to the seven-year civil war in Syria.
Mattis told the Manama Dialogue that Moscow's "opportunism and willingness to overlook Assad's criminal activities against his own people evidences its lack of sincere commitment to essential moral principles."
Iran's support for the Syrian regime "coupled with Russia's repeated vetoes of UN Security Council Resolutions, is the leading reason Assad remains in power," he said.
More than 360,000 people have been killed since the conflict erupted in 2011, while millions have been displaced, many of them seeking refuge in neighbouring countries such as Jordan, Turkey and Lebanon.
"We are deeply aware of the sacrifices many of our partners have made, and continue to make, in dealing with the effects of Assad's violence against his own people," Mattis said.
He vowed the US-led coalition in Syria would "continue to root out" jihadist groups "and expand space for our diplomats to negotiate for long-term peace in that war-torn country".
"We stand with our partners who favour stability over chaos, and we support unity of effort among our nations' militaries in response to shared threats and challenges," Mattis added.
Turkish President Recep Tayyip Erdogan was to host Putin, French President Emmanuel Macron and German Chancellor Angela Merkel in Istanbul for talks on the Syrian conflict.
It will be the first summit to bring together the Turkish and Russian leaders with the European Union's two most significant national leaders.
Turkish presidential spokesman Ibrahim Kalin said on Friday that the primary goal would be to "clarify the steps to be taken for a political solution and to determine a roadmap".