Major backers of Syria's warring rivals will seek to narrow divisions over the future of the country and its embattled President Bashar al-Assad.
The US, Russia, Saudi Arabia, Turkey and Iran will meet in Vienna on Friday, with the aim of achieving a political settlement to help end the Syrian war.
The group met for the first time one week earlier, without an Iranian delegation.
Representatives from Britain, Egypt, France, Germany, Italy, Lebanon, the European Union, and possibly other Arab states, are also expected to attend.
"It's a very good start now that we have Iran involved... Everybody has realised that the Syrian crisis cannot be won militarily," Lina Khatib, research associate with the Arab Reform Initiative at SOAS University, told Al Jazeera.
"Unfortunately, the Syrian opposition doesn't have a presence. They should have a say in what happens. They should be involved in the next stage of negotiations," she added.
The meeting's common ground would be the removal of the Islamic State of Iraq and the Levant (ISIL) group, she explained.
US Secretary of State John Kerry met his Iranian counterpart, Javad Zarif, in the Austrian capital on Thursday in a closed-door meeting.
Later on, Kerry met with his counterparts from Russia, Turkey, and Saudi Arabia to work through their differences ahead of Friday's wider talks.
Iran's Zarif was also due to meet Russian Foreign Minister Sergey Lavrov ahead of the fresh round of talks, according to the Russian news wire Interfax.
Tehran joins in what will be the first time it has participated in international negotiations aimed at resolving the crisis, with the focus on the future of Syria's Assad.
The inclusion of Iran - a key Assad backer - marks a crucial shift after it was excluded from earlier discussions, largely due to opposition from the US and Saudi Arabia.
Al Jazeera's Mohammed Jamjoom, reporting from Vienna, said: "One of the key things that will unfold is whether or not the parties will be able to get Assad and the opposition to talk to each other."
But he said the fact the talks were being held at all was being seen as positive.
"To hear it from the few diplomats who spoke to the press today, just the fact that people have gathered here in Vienna is a cause for celebration," he said.
Though Syria's opposition is currently excluded from these talks, a senior Russian diplomat said he had met with representatives of the Free Syrian Army, among other Syrian opposition groups.
Without naming the individuals he had met, Deputy Foreign Minister Mikhail Bogdanov confirmed the meetings took place in Moscow and Cairo, state news agency RIA Novosti reported on Thursday.
Bogdanov added that he also had talks with various other Syrian opposition figures.
Earlier this week, however, Osama Abu Zeid, a senior leader of the FSA, denied media reports that Russia had invited the FSA for dialogue.
Russia has rejected Western criticism of failing to focus its air strikes in Syria on its declared target, ISIL, and targeting moderate rebels fighting Assad's military, such as the FSA.
Bogdanov's claim followed other statements by Russian diplomats, who said Moscow had reached out to various Syrian opposition groups opposed to ISIL.
Escalation of deaths
Meanwhile, the international medical charity Doctors Without Borders has reported an escalation in deaths from air strike attacks on hospitals in Syria over the last month - roughly the same period Russia began its air campaign in the war-torn country.
The group said in a statement on Thursday that 35 Syrian patients and medical staff have died and 72 have been wounded in the attacks in northern Syria's provinces of Aleppo and Idlib and the central province of Hama.
These are also provinces where Russian air strikes have largely been taking place since September 30. Syrian government jets have also continued their attacks in the meantime.
The aid group said 12 hospitals have been targeted in attacks, including six it supports. The attacks have caused five hospitals to shut down.
Also on Thursday, a monitoring group said at least 595 people have been killed by Russian air strikes in Syria nearly a month into Moscow's military campaign.
A Syrian rebel group, meanwhile, part of a new US-backed alliance that also includes the Kurdish YPG militia, announced an imminent offensive against ISIL in Raqqa province, the group's stronghold in Syria.
The Raqqa Revolutionaries Front, which made the announcement in an online video statement, is one of several rebel groups that recently formed the alliance.
The group said earlier this month that US weapons were on their way.
Source: Al Jazeera and agencies