Syria's opposition says it won't attend this week's peace talks in Switzerland, unless the United Nations withdraws an invitation for Iran.
Iran is a key backer of Syrian President Bashar al Assad.
UN Secretary General Ban ki-Moon says Iran has pledged to play a "positive and constructive role" in efforts to end Syria's 3-year-old civil war.
The US also wants Mr Ban to withdraw the invitation, unless Iran gives 'explicit' support to the the conference's aim of setting up a transitional government in Syria.
But Ban ki-Moon says Tehran has promised to make a positive contribution:
Ireland's Foreign Affairs Minister, Tánaiste Eamon Gilmore arrived in Brussels this morning for a meeting of European foreign ministers - he spoke to reporters on his way into the meeting:
The Syrian National Coalition threatened on Twitter this morning to withdraw from the negotiations, due to start on Wednesday in Montreux, unless the invitation was retracted.
The warning came just hours after international leaders had hailed the coalition's decision to take part in negotiations.
Invitation extended
Mr Ban told a news conference on Sunday that he had extended a late invitation after intense talks over two days with Iranian Foreign Minister Mohammad Javad Zarif.
"Foreign Minister Zarif and I agree that the goal of the negotiations is to establish, by mutual consent, a transitional governing body with full executive powers," Mr Ban said.
"He assured me again and again that Iran, if they are invited, then they will play a very positive and constructive role."
40 nations will attend
If Iran does go, there will be 40 countries and a group of regional bodies at the opening meeting, which will be the most intensive diplomatic effort yet to end a war that the UN says has left well over 100,000 dead.
Mr Ban said that, as host of the peace conference, he also extended last-minute invitations to Australia, Bahrain, Belgium, Greece, Luxembourg, Mexico, the Netherlands, South Korea and the Vatican.
He said the additional 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 the opposition and Mr 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 - China, Britain and France - and additional countries suffering fallout from the war, including Iraq, Jordan and Lebanon.
Catastrophic effects
More than 2.3 million people have fled the country and some 6.5 million are displaced inside Syria.
At least 30 people died in violence in Syria on Sunday, including 15 killed when government planes dropped bombs on rebel zones in the northern province of Aleppo, according to the Syrian Observatory for Human Rights.
