International talks in Vienna this week will test whether Russia and Iran are "serious" about finding a political solution to the war in Syria, the Saudi foreign minister has said.

Adel al-Jubeir's comments on Wednesday came after Iran announced it was accepting an invitation by the US and Russia to attend the talks in the Austrian capital.

"If they're serious we will know, and if they're not serious we will also know and stop wasting time with them," Jubeir said at a news conference in Riyadh, the Saudi capital, with visiting British Foreign Secretary Philip Hammond.

The two-day talks will also "test the intentions of the Syrians and the Russians," Jubeir said.

The talks formally begin on Friday but US Secretary of State John Kerry was expected to meet some other participants later on Thursday.

Nearly 20 nations have signalled that they will attend.

'Bridge the gap'

Hammond said the meetings will be a chance to see if it is possible to "bridge the gap" between Iran and Russia on one side, and other countries against Bashar al-Assad, on the opposite side, on the role of the Syrian president. 

While Russia and Iran think Assad should be able to stand for re-election so that the Syrian people can decide on his future, Hammond said all others agree that Assad has "too much blood on his hands" and should go.

Iran's role in Syria once again in focus

Jubeir said the Saudi position had not changed. "There has to be certainty that Bashar al-Assad will leave," he said.

Iran earlier confirmed that Foreign Minister Javad Zarif will join his counterparts from Russia, the US, Turkey, and Saudi Arabia at the talks.

The inclusion of Iran - a key backer of Assad - marks a crucial shift after Tehran was excluded from earlier talks, mainly because of opposition from Washington and Riyadh.

The Vienna discussions are expected to be the first time all major international players in the conflict, linked to the nearly five-year war, will participate.

Russia and Iran are supporting Assad's forces. Saudi Arabia, Turkey and the West are backing rebels fighting Assad.

Iranian losses

In an interview with Al Jazeera, Iranian journalist Ghanbar Naderi said the increasing number of deaths of Iranian soldiers in Syria may have prompted Tehran to join the negotiations.

"The soldiers in Syria are not normal people, they are the top commanders from the IRGC [Iranian Revolutionary Guard Corps] and [Iranian] elite forces," he said.

"Losing these key people in Syria, it is definitely under pressure. I think if Iran is accepting the dialogue, to now sit next to the international community, to find a way out of the Syrian crisis by dialogue and politics, I think it's because it doesn't want to lose any more commanders."

Tehran officially denies it has combat troops in Syria. But there have been reports that 18 high-ranking officers have been killed in Syria in the last three years.

Iran has also spent billions of dollars in the past four years to keep Assad in power, with hundreds of its troops on the ground.

Renad Mansour, a visiting scholar at the Carnegie Middle East Centre in Beirut, said dialogue with Iran is essential to end the war in Syria.

"Iran has now accomplished the nuclear issue, which was the top priority and now they're moving on," he told Al Jazeera.

"And Iran isn't happy with the situation in Syria. It wants to move away from being a pariah [state]." 

Source: Al Jazeera and agencies