A conference on ending the fighting in Syria will not happen in June, as planned, because both sides in the conflict are unprepared to attend, the UN's international envoy Lakhdar Brahimi said.

Speaking in Geneva on Wednesday, where the international peace meeting on Syria was due to take place this month, Brahimi said: "The Syrian sides are not ready ... the opposition has a lot of work to get ready for this conference."

The US and Russia had spearheaded the meeting, with Washington taking the lead on bringing the opposition to the table, and Moscow working on getting the government of President Bashar al-Assad to attend.

"There is still a lot of work to do to bring a conference about. For this reason, we have come to the conclusion today that it will not be possible to hold this conference in June," said Brahimi.

The meeting might be held in July, he said, after preparatory talks with US and Russian officials.

He will chair a second round of preparatory talks on June 25.

"I think it is embarrassing for us that we are not capable of holding this conference already," Brahimi said. "The opposition has to complete a lot of work to get ready for this conference. I think that they are doing that. And until the opposition is ready, all we can do is wait. Of course I encourage them, push them, help them get there, but obviously the conference can’t take place without them."

Participation dilemma

Brahimi also said that the delay is not a sign of "sticking points from above", meaning the postponement is solely because of warring Syria parties.

Earlier on Wednesday, reports said Russian, US and UN officials meeting in the Swiss city failed to resolve questions over who would take part.

"The most difficult question is the circle of participants in the conference," said Russian Deputy Foreign Minister Gennady Gatilov, according to Interfax news agency.

He also said peace talks would not happen in June.

"The whole issue is that the Syrian opposition, unlike the government, has not made a fundamental decision about its participation in this conference," Gatilov said.

The idea for 'Geneva 2' came from the US and Russia, two countries which do not agree on most things concerning Syria. But for the first time in months, they agreed at the beginning of May on a plan for the way forward.

"Geneva 2 would not be just another international meeting on Syria. What is proposed is face-to-face talks between one delegation representing the opposition, and another made up of regime figures," says Al Jazeera's diplomatic editor James Bays.