Geneva, Switzerland - UN-led negotiations on the war in Syria got off to a delayed start following disputes over the participation of the Syrian opposition delegation.

Opposition representatives nearly missed the opening ceremony of the talks on Thursday after threatening not to attend over disagreements on the make up and format of the session. But in a last-minute turnaround, they arrived late and as one large delegation.

UN envoy Staffan de Mistura formally began the fourth round of talks in Geneva in an opening session that brought the opposition and government delegations face-to-face at UN headquarters with expectations of a breakthrough low.

"I ask you to work together. I know it's not going to be easy to end this horrible conflict and lay the foundation for a country at peace with itself, sovereign and unified," de Mistura told the two delegations, who sat on opposite sides of the stage.

"It is your opportunity and solemn responsibility … not to condemn future generations of Syrian children to long years of bitter and bloody conflict."

The talks are part of the latest political initiative to bring an end to a six-year war that has killed nearly half a million people, wounded more than a million, and forced more than 12 million - half of the country's prewar population - from their homes.

Hopes for a 'work plan'

In a news conference shortly after his opening speech, de Mistura said he would meet each side on Friday in the hope of setting a "work plan" for the remainder of the negotiations.

At the last Syria talks in Geneva 10 months ago, de Mistura had to shuttle between the government and opposition delegations in different rooms.

The opening ceremony on Thursday was delayed by several hours after disputes between the main opposition bloc - the High Negotiations Committee (HNC) - and de Mistura over the structure of the opposition delegation.

The bloated size of the delegation was due partly to de Mistura's inclusion of two other groups - the Moscow and the Cairo platforms - in the talks. The envoy invited the two pro-Russia, government-tolerated opposition groups to sit separately from the HNC, an umbrella group of armed and political factions.

"You must have seen that there was, in particular, a very heavy [presence] on the side of the opposition in the room … they were including also the armed groups … because, as you know, peace is made between those who fight each other," said de Mistura.

The idea of the opposition sitting at different tables riled the Saudi Arabia-based HNC, leading to hours of last-minute diplomacy before the opening ceremony as diplomats scrambled to find a solution.

"Today, the real opposition that represents the Syrian people is the HNC. This delegation and the HNC, extends its hand to any national partner that adopts the will of the Syrian people," Naser al-Hariri, head of the HNC delegation, told reporters before the opening session.

READ MORE: Syria's Geneva talks - 'Expectations pretty low'

"We hope that the Moscow and Cairo platforms will prioritise national interest and the interests of the Syrian people," Hariri said.

"The HNC was in contact with the Cairo and Moscow platforms in previous meetings. There are ongoing efforts to join these platforms within the opposition delegation so that we are represented as one delegation."

De Mistura said there had been "serious progress" made in the hours leading up to the opening ceremony in "forming a united political opposition," but that there was still much work to be done.

Truce violations

The talks in Geneva came about after Russia, a key ally of Syrian President Bashar al-Assad, and Turkey, a backer of the Syrian opposition, managed to forge a fragile nationwide ceasefire in place since December 30.

The Syrian government and the opposition agreed to participate in negotiations despite daily violations of the truce.

Much has changed on the ground in Syria since de Mistura suspended the last round of talks in Geneva last April after a previous ceasefire collapsed and heavy fighting resumed.

Russia’s September 2015 military intervention drastically changed the balance of power, propping up Assad’s embattled forces and helping them to retake key parts of the country.

With the help of Russian jets and Iranian-backed fighters, Syrian government forces dealt the rebels their biggest defeat in the conflict in December by retaking Aleppo, Syria’s commercial capital before the war and a rebel stronghold since 2012.

The Russian-backed push on the battlefield has been coupled with a similar takeover by Moscow in the diplomatic arena - a move helped by confusion surrounding US President Donald Trump’s Syria policy.

While the Geneva talks are seen as the most serious diplomatic effort in months, disputes over the agenda and long-standing disagreements between the opposition and the government on the future of the country have cast doubts on whether any progress will be achieved.

A day before the talks began, de Mistura said he was not expecting any major breakthroughs, but added he was determined to maintain "proactive momentum" on UN Security Council Resolution 2254, a document that provides the backbone of the talks.

"2254 lays out a clear agenda, including specific language on governance, constitutions, elections, and even for the way negotiations should be timed," said de Mistura. "That is what must now be discussed."

Though matters on the ground have shifted, the starkly different political objectives of the warring sides remain unchanged from previous rounds of negotiations.

For the Syrian opposition, a political transition that ensures the removal of Assad remains the only option for peace - an issue that the government in Damascus has consistently refused to consider.

De Mistura said the biggest challenge before the delegates was a "lack of trust" as he appealed to the two sides to use the talks as an opportunity for peace.

"We do know what will happen if we fail once again - more deaths, more suffering, more terrorism, more refugees," he said.