The UN has withdrawn its invitation to Iran to join this week's peace conference on the Syria crisis.
UN spokesman Martin Nesirky criticised Tehran for failing to back the plan to form a Syrian transitional government, which is the basis of the conference.
The invitation to Iran, a key ally of the Syrian regime, angered the US and the Western-backed Syrian opposition.
The peace conference, due to begin on Wednesday, is the biggest diplomatic effort to end the three-year conflict.
More than 100,000 people have been killed and millions more displaced in the war.
The Syrian government and the main exiled opposition group, the National Coalition, are due to send delegates to the conference, which begins in the Swiss town of Montreux.
The National Coalition had threatened to pull out if the invitation to Iran was not rescinded, but they have since confirmed that they will now be attending.
Withdrawing the invitation was "the right thing to do", Monzer Akbik, the National Coalition's chief of staff, told the BBC.
Mr Nesirky said UN chief Ban Ki-moon had invited Iran after speaking in private to senior Iranian officials, who had assured him that they "understood and supported the basis and goal of the conference".
But Iran issued several statements on Monday rejecting any attempt to place conditions on its attendance at the conference.
Mr Nesirky said: "Given that it has chosen to remain outside that basic understanding, he has decided that the one-day Montreux gathering will proceed without Iran's participation."
It is unclear whether Iran will be able to join the talks two days later, when they move to Geneva.
US officials welcomed the withdrawal, saying they hoped the focus could return to the content of the talks.
Meanwhile, CNN and UK newspaper the Guardian are reporting claims that the Syrian regime tortured and killed thousands of detainees.
The BBC's Jim Muir in Beirut says if the account is to be believed, it shows a chilling systematic documentation of the bodies, each of which was photographed several times and given a number.
Some 55,000 photographs showing roughly 11,000 dead detainees were smuggled out of Syria by a defector who served as a military police photographer, according to CNN.
The Guardian suggests that the publication of the evidence, along with a Qatar-funded report scrutinising the credibility of the evidence, appears to have been timed deliberately to coincide with the peace conference.
The conference is the culmination of months of diplomacy.
In May last year, Russian Foreign Minister Sergei Lavrov and US Secretary of State John Kerry agreed to try to bring both sides together.
Later, the UN Security Council called for a conference to implement the Geneva communique - a deal on a transitional government agreed at a UN-backed meeting in 2012.
However, the National Coalition appears resolute that any transitional government will not involve President Bashar al-Assad.
For his part, Mr Assad said in an interview on Monday that the possibility of the National Coalition obtaining any ministerial positions in a new government was "totally unrealistic".