Syria peace talks were in disarray on Tuesday before they began, buffeted by a botched UN invitation to Iran and an explosion in Beirut.

UN Secretary-General Ban Ki-moon's last-minute decision on Sunday to invite President Assad's main foreign backer Iran - only to withdraw the invitation a day later - is expected to undermine talks that are already given little chance of success, according to Reuters.

A suicide bomber killed four people in Beirut, capital of Lebanon, showing the urgent danger of sectarian violence spilling to Syria's neighbors, three years into a civil war that has already killed at least 130,000 people inside Syria.

The peace conference set to begin on Wednesday will include the first talks between Assad and his opponents. But hopes of a breakthrough are negligible at a time when fighting has escalated and neither side shows any sign of retreating from its demands or being able to end the war with a victory.

Around a third of Syria's 22 million people have been driven from their homes, many to refugee camps abroad; half are in desperate need of international aid.

The country at the heart of the Middle East has been carved up on ethnic and sectarian lines, with neighbors and distant powers lining up to arm and fund rival factions.

It has been 18 months since a previous international peace conference in Geneva ended in failure, and all other diplomatic initiatives have also proven fruitless.

"At best, Geneva II will reconfirm agreements made during the first Geneva conference, call for ceasefires, maybe prisoners swap and so on," said one Western diplomat.

The opposition says the talks, actually taking place in Montreux on Wednesday, must seek Assad's removal from power.

The opposition team due to arrive in Switzerland on Tuesday is headed by Ahmed Jarba, leader of the opposition National Coalition, which only agreed to attend at the last minute and nearly pulled out on Monday over Ban's invitation to Iran.