BEIRUT/WASHINGTON: Iran will take part in international talks on Syria for the first time this week, giving it a voice in the effort to find a resolution to the nearly 5-year-old civil war that has so far defied even the slightest progress toward peace.

A crucial backer of Syrian President Bashar Assad, Tehran has been shunned from all previous talks on Syria. Its inclusion now marks recognition by the United States that no discussion on Syria’s future can succeed without Iran at the table.

News of Iran’s attendance outraged Syrian rebels, who said its participation will only prolong the conflict. The gathering, which takes place Thursday and Friday in Vienna, will also put Iran in the same room with its most bitter regional rival, Saudi Arabia, raising the potential for tensions.

Saudi Arabia said it aimed to gauge during the talks the willingness of Iran and Russia, the main backers of Assad and his government, for a peace deal, Foreign Minister Adel al-Jubeir said 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,” 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.

U.S. Secretary of State John Kerry warned Wednesday that the Vienna talks will not find an immediate political solution but nevertheless represent the best hope available.

“While finding a way forward on Syria will not be easy – it’s not going to be automatic – it is the most promising opportunity for a political opening we have seen,” he said in a speech on Middle East policy just before he was to set off for Vienna.

“My friends, the challenge that we face in Syria today is nothing less than to chart a course out of hell.”

At the heart of the Vienna talks – and the most contentious issue – is the future of Assad.

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.

Jubeir said Saudi Arabia and its allies would hold a separate meeting Friday to seek “the time and means of Bashar 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.

It was not clear whether any invitation had been issued to either the Syrian government or the opposition.

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.

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,” the coalition’s Vice President Hisham Marwa said.

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.”

Turkey has no objection to Iranian participation in the Syria talks, a diplomatic source in Ankara said.

Iran’s participation reflects its newfound place in the international community following the nuclear deal reached with world powers earlier this year. It also shows the seismic shift brought about by Russia’s direct military involvement in Syria since launching a campaign of airstrikes on behalf of Assad last month. That intervention has emboldened Assad’s supporters.

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 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 jihadis.”

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.