Syrian opposition negotiators are demanding the release of thousands of detainees, including hundreds of women and children being held by Bashar al-Assad’s government, as a confidence-building measure before joining UN-brokered peace talks in Geneva.

Syrian and western sources said on Saturday that an initial list of 177 prisoners had already been given to Staffan de Mistura, the UN envoy trying to restart long-stalled negotiations between bitter enemies nearly five years into the conflict.

In total, the opposition is seeking the release of 3,000 people, including 700 held in Deraa, the largest city in southern Syria. “We are trying to push for women with newborn children,” said Farah Atassi, an opposition adviser. “That’s a priority. Then we will move to the next list.” In the past the government has pledged to free political detainees but in the end released only ordinary criminals.

The Geneva talks got under way on Friday with a meeting between De Mistura and a Syrian government delegation. The opposition announced later that it would also attend after a day of confusion and pressure – and receiving assurances from the US and UN that its demands for an end to airstrikes and improved humanitarian access would be taken seriously.

UN special envoy Staffan de Mistura speaks to the media after meeting with the delegation of Syrian government at Palais des Nations in Geneval. Photograph: Xu Jinquan/Xinhua Press/Corbis

John Kerry, the US secretary of state, made the pledge to Riyadh Hijab, head of the opposition’s higher negotiations commission, after being accused of angering him by speaking bluntly in a stormy meeting last weekend. Adel al-Jubeir, the Saudi foreign minister, also helped to persuade the reluctant Syrians to take the plunge, diplomats said.

The anti-Assad rebels are pressing for immediate implementation of two clauses of UN resolution 2254, which underpins the latest phase of international diplomacy on Syria. These address humanitarian issues and call for an end to attacks on civilians.

The 17-strong opposition team flew to Geneva on Saturday on a private plane arranged by Saudi Arabia. The Syrians – representing both political and armed groups – spent four days in Riyadh debating whether to turn up, and were hailed by supporters on social media for standing firm and insisting that their conditions were met.

Formal sessions of what the UN is calling “proximity talks” – separate meetings between De Mistura and each Syrian faction – are scheduled to take place in the Palais des Nations on Monday, a full week after the negotiations were originally scheduled to begin. No direct contact is expected.

“We are going to Geneva to put to the test the seriousness of the international community in its promises to the Syrian people, and to also test the seriousness of the regime in implementing its humanitarian obligations,” said an opposition spokesman. “We want to show the world our seriousness in moving toward negotiations to find a political solution.”

Diplomats say freeing prisoners is more easily achievable than ceasefires and allowing aid convoys into 15 locations defined as under siege across the country. Releases will also allow the opposition to demonstrate to sceptical supporters that it has concrete gains to show for taking part in the talks – and help prepare the ground for taking tougher decisions.

UN special envoy Staffan de Mistura speaks to Syria’s ambassador to the UN Bashar al Jaafari during Syria peace talks in in Geneva. Photograph: Reuters

Kerry’s assurances to the Syrian opposition have not been made public, but they are believed to have included a promise to look at options for breaking sieges, most likely using air drops of food and other supplies. Overall, however, there is a growing mood of mistrust of the US, with European and Arab diplomats saying privately that they believe Washington is playing for time for the remaining months of Barack Obama’s term in the White House.

De Mistura also plans to convene four standing UN working groups, looking at humanitarian issues; security, including efforts to fight Isis; political and legal matters; and reconstruction. Diplomats describe an attempt to create a permanent forum for negotiations, avoiding direct confrontation and collapse if possible. “He is trying to produce a slower process that becomes a normal form of engagement with a bit of flexibility,” said one official.

An estimated 250,000-300,000 people have been killed and 10 million displaced since the Syrian crisis erupted in March 2011. Two previous Geneva conferences failed to halt the war.

