The United Nations has cancelled an invitation to Iran to attend a Syria peace conference this week over its refusal to back calls for a transitional government in Syria.
UN Chief Ban Ki-moon is "deeply disappointed" at Iran's statements rejecting a communique adopted by international powers on ending the Syria war, said UN spokesman Martin Nesirky.
Syria's western-backed opposition National Coalition confirmed it would take part in international talks.
"We appreciate the United Nations and (UN Secretary General) Ban Ki-moon's understanding of our position.
"We think they have taken the right decision. Our participation is confirmed for 22 January," Monzer Akbik, chief of staff of the president of the National Coalition told Reuters.
Another coalition member, Anas Abdah, said they would send a list of conference delegates to the United Nations later.
The Syrian National Coalition only decided on Saturday to attend the conference and reacted with fury to the UN's invitation.
Earlier Coalition spokesman Louay Safi announced on the coalition's Twitter account that the opposition group would withdraw "unless Ban Ki-moon retracts Iran's invitation".
The threat came only hours after international leaders had hailed the coalition's decision to take part in negotiations.
Iran pledged to play a "positive and constructive role" in efforts to end the war.
It said it had accepted the invitation to attend the talks and would do so without preconditions.
Mr Ban said he made the late invitation after intensive talks over two days with Iran's Foreign Minister Mohammad Javad Zarif, who agreed to attend.
The UN Secretary General said heated discussions were under way following the opposition threat.
"Intensive and urgent discussions are under way and I'll have more to say about the situation later in the day," Mr Ban said.
"For the moment, let me just appeal again to all involved to keep the needs of the Syrian people foremost in mind."
The United States and other Western powers had also opposed Iran's attendance at the meeting as long as it refused to accept a communique adopted by the major powers in Geneva last year calling for a transitional government in Syria.
Earlier, Russian Foreign Minister Sergei Lavrov said the absence of Iran from the talks would be an "unforgivable mistake".
"Not to ensure the presence at this event of all those who may directly influence the situation, I think, would be an unforgivable mistake," said Mr Lavrov.
Talks between President Bashar al-Assad's government and the opposition are due to start in Geneva on Friday.
Mr Ban said that as host of the peace conference, he also made last-minute invitations to Australia, Bahrain, Belgium, Greece, Luxembourg, Mexico, Netherlands, South Korea and the Vatican.
He said the extra countries would be "an important and useful show of solidarity in advance of the hard work that the Syrian government and opposition delegations will begin".
The US and Russia played key roles convincing pressing the opposition and President Assad to send delegations to the conference, which has been pushed back several times.
They will attend alongside the other permanent members of the UN Security Council including China, Britain and France and additional countries suffering fallout from the war, such as Iraq, Jordan and Lebanon.
Mr Ban said it was vital for the ministers at the Montreux meeting to urge the Syria protagonists "to act and negotiate in good faith for their own future".
"This violence must stop for humanity and for the future of Syrians," he said.
The UN has launched its biggest ever operation to help refugees from the conflict and millions inside Syria who have been left homeless and without food.
More than 2.3 million people have fled the country and about 6.5 million are displaced inside Syria.
Assad may seek new term
Meanwhile, President Assad has said there is a "significant chance" he will run in the presidential elections in June.
If "there is public desire and a public opinion in favour of my candidacy, I will not hesitate for a second to run for election".
"In short, we can say that the chances for my candidacy are significant," added Mr Assad, who succeeded his father Hafez in 2000.
Speaking at the presidential palace in Damascus, Mr Assad also said he expected his country's bloody conflict to drag on, calling it a "fight against terrorism".
In an interview with AFP, he dismissed the opposition as having been "created" by foreign backers.
He described the possibility of appointing key opposition figures to the post of prime minister as nothing more than "a good joke".
They "come to the border for a 30-minute photo opportunity and then they flee," he said. "How can they be ministers in the government?"
"These propositions are totally unrealistic, but they do make a good joke!"
The Syrian leader said he expected the country's conflict to grind on, although he said his forces were making progress.
"What we can say is that we are making progress and moving forward. This doesn't mean that victory is near at hand; these kinds of battles are complicated, difficult and they need a lot of time," he said.
"Should Syria lose this battle, that would mean the spread of chaos throughout the Middle East."
Mr Assad said peace talks should focus on his "war on terrorism", despite the opposition's insistence the talks would lead to his departure from office.
"The Geneva conference should produce clear results with regard to the fight against terrorism in Syria," he said.
"This is the most important decision or result that the Geneva conference could produce. Any political solution that is reached without fighting terrorism has no value."
Syria guilty of 'industrial-scale killing'
Three former international prosecutors have accused Syria of large-scale killing and torture in a report - based on evidence of a defector - commissioned by Qatar, which backs Syrian rebels, the Guardian and CNN has reported.
The report, which can be found online at both news outlets, relies on the unauthenticated testimony and photographs given by the source, who remains unidentified for security purposes.
The informant, a photographer who claims to have defected from the Syrian military police, presented forensic experts commissioned by the London legal firm representing Qatar with around 55,000 digital images of 11,000 dead detainees.
He claims they died in captivity before being taken to a military hospital to be photographed.
"Overall there was evidence that a significant number of the deceased were emaciated and a significant minority had been bound and/or beaten with rod-like objects," said the report.
Some had no eyes while others showed signs of strangulation or electrocution, according to the 31-page document.
The report was written by Desmond de Silva, former chief prosecutor of the special court for Sierra Leone; Geoffrey Nice, the former lead prosecutor in the trial of former Yugoslavian president Slobodan Milosevic; and David Crane, who indicted Liberian president Charles Taylor.
Mr De Silva told the Guardian that the evidence provided the "smoking gun" proving that the regime of President Assad had carried out "industrial-scale killing".
Mr Crane called the evidence "amazing" and suggested there was a strong case for prosecution.
"Now we have direct evidence of what was happening to people who had disappeared," he explained.
"This is the first provable, direct evidence of what has happened to at least 11,000 human beings who have been tortured and executed and apparently disposed of."
The authors said they found the informant and his evidence to be credible after subjecting them to "rigorous scrutiny" and have made their findings available to the United Nations, governments and human rights groups.
Sunni Qatar was quick to back rebels who rose up in 2011 against the rule of Assad, who is backed by Shia powerhouse Iran.
Tánaiste calls for humanitarian assistance
Tánaiste and Minister for Foreign Affairs Eamon Gilmore has said he will be pressing hard for more to be done to resolve the humanitarian crisis in Syria during discussions with his EU colleagues in Brussels.
Mr Gilmore said: "I think there are many people who are questioning how inspectors can get through to deal with chemical weapons, but yet the humanitarian aid is not getting through to the people who need it."
The Tánaiste said he believed a stronger resolution was needed at the UN to address the problem, and he would be asking his EU partners to support such a move.
Mr Gilmore said there were "huge numbers in need of food, medicine and shelter".