* First time Iran, Saudi attend multilateral Syria talks
* UK foreign minister sees hope for dialogue between rivals
* Syrian opposition against Iranian participation
* Iran faces first diplomatic test since nuclear deal (Edits, adds White House, experts)
By Angus McDowall and Bozorgmehr Sharafedin
RIYADH/DUBAI, Oct 28 (Reuters) - Arch-rivals Saudi Arabia and Iran announced they would attend international talks in Vienna on Friday on the Syrian conflict, in what will be their first meeting to discuss the four-year-long war.
Saudi Arabia said it aimed to gauge during the talks the willingness of Iran and Russia, the main backers of Syrian President Bashar al-Assad and his government, for a peace deal, Foreign Minister Adel al-Jubeir said on Wednesday.
"The view of our partners ... was that we should test the intentions of the Iranians and the Russians in arriving at a political solution in Syria, which we all prefer," al-Jubeir told a news conference in the Saudi capital Riyadh.
Iranian Foreign Minister Mohammad Javad Zarif and three of his deputies will travel to Vienna, Iranian state news agencies said. It will be the first time that Tehran has been represented in international discussions on the Syrian crisis.
Iran says it supports a political solution in Syria, but says Assad should be part of the process. Opposition groups, and their regional backers including Saudi Arabia, Qatar and Turkey, say Assad must leave power as a precondition for peace.
Al-Jubeir said Saudi Arabia and its allies would hold a separate meeting on Friday to seek "the time and means of Bashar al-Assad's exit".
The White House said the peace talks could only work if "all key stakeholders" were invited, adding that Iran's participation should not overshadow the efforts to end the Syrian crisis.
"The United States is prepared to work with any nation, including Russia and Iran, to resolve the conflict in Syria," White House spokesman Eric Schultz told reporters.
Egypt, Iraq, Qatar, Lebanon, the European Union and France also said they would attend Friday's talks, which come a day after a smaller round of negotiations between the United States, Russia, Saudi Arabia and Turkey.
Around a dozen participants are expected in total.
It was not clear whether any invitation had been issued to either the Syrian government or the opposition. Neither side was present at the last talks in Vienna.
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Speaking at a news conference in Riyadh with his Saudi counterpart, British foreign minister Philip Hammond said he hoped the meeting would encourage dialogue between the rivals, who back opposing sides in conflicts across the Arab world.
"Saudi Arabia and Iran are the two most important and powerful countries in this region. It is very much in the long-term interests of the region that eventually these two countries are able to talk to each other, are able to discuss differences, are able to seek solutions peacefully," Hammond said.
The Syrian National Coalition, an opposition group based in Turkey and backed by Western powers, said Iran's participation in the talks would undermine the political process.
"Iran has only one project - to keep Assad in power ... They don't believe in the principle of the talks," said the coalition's vice-president Hisham Marwa. ID:nL8N12S2Q6
The secretary of Iran's Supreme National Security Council, Ali Shamkhani, told reporters during a visit to Pakistan that Tehran would join the Syria talks "with no pre-conditions".
A senior Western diplomat in New York said on Wednesday: "We have got to start from a fairly low base given that you've got Russia and Iran on one side and everyone else on the other."
"It would be successful if (the meeting) came off and didn't completely fall apart," he added.
Turkey, which backs the anti-Assad opposition, has no objection to Iranian participation in the Syria talks, a diplomatic source in Ankara said. Turkish foreign ministry officials were not immediately available for comment.
IRAN'S FIRST TEST
Fighters from Iran and its Lebanese ally Hezbollah are currently taking part in ground offensives being waged by the Syrian army and its allies with Russian air support.
Revolutionary Guards Deputy Commander Hossein Salami said in an interview on state TV on Monday that Iran had increased its military presence in Syria in recent months to help its army in response to a request from the Assad government.
An Iranian political analyst based in Frankfurt, Ali Sadrzadeh, said: "Iran was always saying that without it the talks on the Syrian crisis would not succeed. What has changed is that Russia and the United States have come to the same conclusion."
He said the July nuclear deal between Iran and world powers paved the way for Tehran's participation in the international arena, adding: "The Vienna talks will be Iran's first test."
EU foreign policy chief Federica Mogherini, who will take part in the talks, welcomed Iran's participation. After a phone call with the Iranian foreign minister on Wednesday, she tweeted: "Important to have all relevant regional actors at the table on Friday in Vienna."
"This is an acknowledgement of reality, four years into the conflict," said Julien Barnes-Dacey, senior policy fellow at the European Council on Foreign Relations in London. "Having Iran at the table complicates the goal of getting rid of Assad, but potentially opens the door to some kind of de-escalatory track."
Some analysts suggested compromise remained far off.
Karim Sadjadpour, senior associate at Carnegie Endowment for International Peace, said: "There are no tangible signs that Iran is prepared to abandon Assad, or sees a way to preserve its interests in a post-Assad Syria."
"Iran has consistently argued that Syria's future is a choice between Assad and the jihadists."
Patrick Clawson, a senior fellow at the Washington Institute for Near East Policy, said the idea that external actors were key to producing a political solution was misplaced.
"The difficulty we face is that the involvement of Iran is only going to inflame the opinion, the attitudes of some groups in Syria," he said.