Geneva (AFP) - The UN struggled on Friday to get a new round of Syrian peace talks off the ground, but with few signs of progress as dozens more civilian deaths underlined the scale of the challenge.

The UN's Syria envoy, who brought rival regime and opposition delegates symbolically together late Thursday, held separate meetings with them Friday to hammer out the talks' format.

But there appeared to be no discussion of substance, either with UN envoy Staffan de Mistura and certainly not between the rival parties themselves.

"We discussed issues relating to the format of the talks exclusively," said Syrian regime delegation chief Bashar al-Jaafari after meeting de Mistura. The opposition said the same after its talks with the UN envoy later.

During three previous rounds of talks in Geneva last year, the rivals never sat down at the same table, instead leaving de Mistura to shuttle between them.

At the end of the day de Mistura's acting chief of staff Michael Contet signalled there was no immediate prospect of direct talks.

"Of course he wishes to be able to have all the invitees present in Geneva in the same place to exchange directly and have direct talks," he told reporters.

But "at present there are difficulties for some of them to engage in such a format... This is why the special envoy will be continuing his efforts so that things can evolve towards direct engagement," he added.

- Over 300,000 dead -

Even as the new UN talks began, the death toll in a suicide bombing near the Syrian town of Al-Bab rose to 51, the latest atrocity in a six-year war which has killed more than 310,000 people.

Most of the dead in the attack -- claimed by the Islamic State (IS) group -- were Turkish-backed rebels, who had only just taken the stronghold town from IS militants.

In addition two Turkish soldiers were killed, while separately officials in Baghdad said the Iraqi air force struck IS members in neighbouring Syria.

The attack has no direct bearing on the UN talks, since the IS is not part of the latest ceasefire deal, but it illustrates the lack of any return to normality for war-ravaged Syria.

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In Geneva, de Mistura -- hosting the first UN-sponsored talks since April -- acknowledged the frailty of the latest ceasefire, which was agreed in late December.

- 'Not expecting miracles' -

In his welcome address, the envoy played down hopes of a breakthrough.

"I'm not expecting miracles," he admitted while warning of dire consequences if the talks "fail again".

The talks between negotiators for Syrian President Bashar al-Assad and the opposition have been clouded by persistent violence and deadlock over the country's political future.

The main opposition High Negotiations Committee (HNC) has said it wants to meet the government face-to-face.

But on the ground rebels are in a significantly weaker position since the last UN-sponsored round.

The army has recaptured the rebel bastion of eastern Aleppo and Washington, once staunchly opposed to Assad, has said it is reassessing every aspect of its Syria policy under President Donald Trump.

- Syrian 'transition' disputed -

The regime delegation chief said Friday that de Mistura had given them a "paper" -- which according to a source close to the talks covers three areas for discussion: transition in Syria, a constitution and elections.

Political "transition", part of UN Security Council Resolution 2254 which managed the Geneva talks, is at the heart of the debate.

But it has different meanings for Damascus and its Russian and Iranian allies on side, and the Syrian opposition on the other.

"As the Russians and the regime see it, a government of national unity is put in place, Bashar al-Assad remains president and they bring in opposition who will look after hunting and sports.

"For the opposition, it's clear that the Syrian president cannot remain in power," said a western diplomatic source.

In New York on Friday the UN Security Council met behind closed doors to discuss chemical weapons use in Syria, as the US, France and Britain pushed for sanctions on Damascus.

But afterwards Russia's deputy ambassador confirmed -- as expected -- that Moscow will use its veto to block a draft resolution imposing sanctions on the Assad regime.