(Adds statement by Britain's Hague)
By Stephanie Nebehay
GENEVA, June 14 (Reuters) - World powers are working towards holding a crisis meeting on Syria in Geneva on June 30 to try to get a tattered peace plan back on track, diplomats said on Thursday, although Britain said the idea of Iran being in the meeting was "probably unworkable".
Kofi Annan, the United Nations-Arab League mediator, has called for convening the Contact Group as soon as possible but the United States opposes the involvement of Iran, Syria's main regional ally.
"It is not confirmed but people are still working toward something on the 30th," a diplomat told Reuters.
Another said: "We're moving toward the 30th but nothing is confirmed."
The participation of Iran was believed to be a sticking point in organising the meeting, a third said.
British Foreign Minister William Hague - who held talks on Thursday with his Iranian counterpart Ali Akbar Salehi on the margins of a conference in Kabul - appeared to rule out Iran taking part in the Contact Group.
Britain's Foreign Office said that Hague reiterated his welcome in principle for an international conference on Syria, which should set out the principles of a political transition.
"He underlined British concern that the possibility of Iranian attendance at any such meeting was probably unworkable," it said in a statement.
There was no immediate comment from Tehran and calls to Iran's diplomatic mission in Geneva were not returned.
Annan's spokesman Ahmad Fawzi declined to confirm any tentative arrangements.
"The Joint Special Envoy (Annan) is currently engaged in urgent and intensive consultations with member states in order to reach consensus on the shape and formula of the Contact Group meeting," he told Reuters.
Issues under discussion include the date, venue and list of participants, he said, without elaborating further.
Proposals to hold such a meeting have gathered pace as violence in Syria has escalated in recent weeks, with the United States and many European countries frustrated by Russian support for Syrian President Bashar al-Assad despite evidence that government-backed forces have carried out massacres.
Russia and China blocked two attempts to agree a U.N. Security Council resolution to put pressure on Syria last year, forcing Assad's critics to look for other ways to bring an end to the violence.
An uprising against Assad's rule began in March 2011 with peaceful demonstrations for democracy but has developed into something close to civil war.
All sides signed up to Annan's six-point peace plan, but an April 12 ceasefire at the centre of his proposal has been widely violated, prompting many observers to say the plan has failed.
(Reporting by Stephanie Nebehay, Writing by Tom Miles; Editing by Angus MacSwan)